1) Keep in mind the standard rules for Psionics-Magic Transparency. Psionics is generally treated as equivalent to magic, and vice versa. Dispel Magic can dispel psionics just as easily and at the same time, Detect Psionics can detect magical auras as well as psionic ones, Antimagic Field can nullify psionics just as well as magic, Dwarves' racial bonus on saves against magic extends to saves against psionics, and so on and so forth. This is the norm in D&D. While I prefer to use the optional rule in the book for 'Psionics are Different', it is only an option, not the norm. If you haven't used psionics enough in the past to be comfortable handling a distinction between the two, then just use the standard transparency rule as normal.
2) Just to note, the book Complete Psionic has some official errata on certain powers from the Expanded Psionics Handbook, which helps to fix some of the problem powers. Also, Complete Psionic has more material for using psionics and playing psionic characters. There are some useful psionic prestige classes, especially for making Soulknives more effective (since they're normally kinda gimped and weak), and it has a handful of new 20-level 'core' classes.
It includes the Lurk class (a psionic rogue type), the Ardent class (a psionic.....er....I dunno exactly, but it uses auras and is kinda like a bard or marshal or dragon shaman in function, I think), the Divine Mind class (a psionic individual who receives their powers from divine inspiration, sort of a psionic cleric, they gain psionic mantles that are kinda like a cleric's domains), and the Erudite class (a powerful psionic manifester who can learn all kinds of powers, sort of a psionic wizard in its studious nature and whatnot, but this is generally considered an overpowered class because of its ability to learn tons of powers and even many unique powers from other class' lists).
3) Always, always, ALWAYS keep in mind the limit on power point expenditures!! NO manifester can spend more power points on a single power than he or she has manifester levels from the appropriate class. A 1st-level Psychic Warrior cannot spend more than 1 power point on any given Psychic Warrior power, for instance. There are only a few ways this limit can be messed with.
The Overchannel feat allows a manifester to get a small, temporary boost to their effective manifester level, but at the cost of taking damage (1d8 for +1 ML, for instance), and the amount of increase/damage is limited by character level (to a maximum of +3 ML at 15th-level onward, for 5d8 damage). The Wilder class has a Wild Surge ability, which works similar to Overchannel but is superior, except that it can potentially daze the Wilder when used, and makes them lose some power points when that happens. The Practiced Manifester feat from Complete Psionic allows a character who's multiclassed as a psionic class and a nonpsionic class to get a boost to their manifester level of up to +4, but it cannot make their manifester level higher than their character level (if I recall correctly; it may've been worded differently, but it's either no higher than CL, or no higher than their number of nonpsionic class levels, or somesuch).
4) Note that psionic manifesters have an advantage in short-range, short-lasting encounters, if you only have one or two encounters per day. They can manifest their best powers more often than a spellcaster can cast their best spells, even though it drains the manifester's power points rapidly. So if you only have a few encounters each day, the psionic folks will be able to go full-power into each, and not have to worry about the fact that they used up all their energy in just 1 or 2 encounters. If you often have more encounters per day, or have much longer encounters, or have occasional surprise encounters at night or the like, a psionic manifester will be equal to other characters or even at a slight disadvantage.
Such encounter frequency will teach them to conserve their power points rather than going 'nova' in every fight/challenge/situation. Psionic powers are often (though not always) possessed of a shorter range and a shorter duration than similar magic spells, with many psionic powers even being 'personal' range rather than touch or close, so psionic manifesters tend to have little in the way of party-buffing, enemy-debuffing, ally-healing, or ally-transport powers. They do still have a little bit of that, I think, but it's scarce by comparison. Also, note that encounters are not necessarily fights; an encounter could be the psion using his powers (and thus, his energy reserves) to deceive some guards and sneak in, or gather information around town, or travel to a distant location, or get past a trap, or get past a barrier/obstruction, or elude a big nasty monster he can't possibly fight, or do some diplomacy and serve as a lie detector/thought-reader.
5) You can also remedy the potential problem of 'nova'-ing, without throwing in more encounters/challenges per day, by imposing a simple house rule. Say, for instance, that a psionic manifester can only access a number of power points equal to his or her highest manifester level (to account for possible multi-classed manifesters) + his or her highest ability score modifier amongst Intelligence, Wisdom, and Charisma. Say that they add the ability modifier again at the 11th manifester level onward. If the available pool of power points, however, will not exceed the character's total maximum power points (so a 1st-level Psychic Warrior with 15 Wisdom will still only have 1 power point).
The rest of their power point reserve is untouchable (to themselves anyway) most of the time; say that they need to spend a full-round action focusing to fill their available pool of power points back to its maximum, transferring points from their reserve. Any power point expenditures would come out of the available pool first, and any excess drained by other sources (not the manifester himself or herself, and not his or her cognizance crystals or psicrystal or whatnot) would be drawn from the otherwise-untouchable reserve. This will keep them from nova-ing in every battle, since they'll have to spend entire turns to refill their available pool of energy after every one or two powers they manifest at full potency (encouraging them to manifest more low-level powers or use less augmentation most of the time). Actions are a precious commodity in high-level play, while low-level psionic manifesters don't really suffer the nova problem much anyway (for instance, a 2nd-level Psion who uses Overchannel to manifest a 3-power-point Energy Ray will be expending half or one-third of his total power points, since low-level manifesters have so very few power points to begin with).
6) As others have mentioned, psionic enemies should not be expending all their power points when the PCs fight them, at least not most of the time. Usually they will not be aware that they're likely to die in that fight if they don't go for maximum power output. Only a few are likely to be familiar enough with the strength of the PCs and know that they will have to pull out all the stops if they're going to escape that fight with their lives intact. The houserule I mentioned above obviates the need for this consideration, but even if not using that houserule, this consideration still deserves remembering.
Just as every high-level wizard is not going to waste his few slots filled with Finger of Death, Flesh to Stone, Prismatic Ray, and Delayed Blast Fireball against every single pathetic fool who decides to challenge his mighty personage, psionic manifesters will not generally go blasting at full-power in every battle. They're likely to gauge the opponents' strength first, using intermediate or weak attacks, before deciding if the nuisance is dangerous enough to warrant full-bore maximum power use.
Also, note that psionic enemies are often a good foil for psionic PCs; you don't have to use 'em, especially since 3.5 removed the distinction of psionic combat modes, but they can make good opposition and nuisances to psionic Player Characters.
7) A few powers have a wierd Augmentation scaling, as others mentioned, where they get +1 damage die AND +1 to their Save DC for each 1 power point spent on augmenting that manifestation. These should be changed to give +1 save DC per 2 power points spent on augmenting the damage dice, like other powers of similar effect do. Most psionic powers have more reasonable augmentation costs and should be fine.
8) You may or may not want to limit the Energy powers a tad. Those which normally allow you to choose an energy type with each individual manifestation. To bring down this flexibility to resemble a spellcaster's limits, you may want to add a requirement that the manifester must choose a single energy type when they learn that power (separate choice for each power), and that the manifester always manifests that power with the chosen energy type. This would prevent on-the-fly manifestations of a Fire Energy Ball against trolls in one encounter of the day and then manifesting a Sonic Energy Ball against demons in the next encounter of the day, for example.
9) Astral Constructs are generally too powerful compared to Summon Monster spells. Complete Psionic has an errata that limits characters to having no more than 1 Astral Construct manifested at once (IIRC), but this doesn't really solve the problem, just reduces the abusiveness of it. I would suggest 2 changes on top of this: first, change the duration of Astral Constructs to "1 round + 1 round/2 levels" and secondly, change the augmentation cost to 3 or 4 power points (normally it's 2pp, but since Astral Constructs are typically more effective than equal-level Summon Monsters....). I'd lean towards 3pp for the augmentation, but I haven't had any opportunity to playtest this yet and see if it's a sufficient cost increase, so 4pp may or may not be more fair.
10) You may want to remove the Schism psionic power altogether. It's a holdover from 3.0 psionics where Schism was kinda the psionic equivalent of Haste, though more limited. The 3.5 version is a bit more limited still, but might be disagreeable to some folks, since it allows an extra power to be manifested each round (even though it has to be a low-level power, and has an increased power point cost), whereas Haste in 3.5 doesn't allow extra spellcasting. However, whether or not Schism is overpowered in 3.5 generally depends on the group; it is higher level than Haste, and personal-range only, but there is some possibility it could be abused. I just don't personally know of any particular abuses of it.
11) Many arguments against psionics are generally along the lines of 'it's too science fictiony', or 'it's too oriental', or 'it's too wierd', or 'it's too anime-ish', which are generally baloney. Such arguments are often broad generalizations that don't quite fit. And psionics can really easily be adjusted in flavor to suit the game-world. Many people use it simply as an alternate magic system, that's just a little more mentalism-oriented and personal-body-oriented than arcane/divine magic. In Eberron it's linked in some way to the Plane of Dreams, Dal Quor. Somewhere in the XPH or elsewhere (I know of at least one reference on the Wizards of the Coast website) it's mentioned that psionics can be treated as the same force behind monkish ki powers and such (or vice versa). Etc.
My only personal problems with psionics in 3.5 are the overemphasis of crystals (stupid new 'crystalpunk' artwork and powers), and the new powers that resemble magical effects too strongly (the newer psionic equivalents of wish, resurrection, etc., which I just don't feel 'fits in' with the rest of the psionic flavor). The main arguments against psionics though are that it's broken, which it is not. Certain elements of it are broken, of course, but there's a lot more broken spells and spellcasting prestige classes around, even in the core rules. For each person who devises a broken psionic combo, you can generally find 2 or 3 people with broken spellcaster or warrior combos. Ergo, there are only a few psionic powers and such that actually need errata, while the rest of the psionic material is pretty reasonable enough as-is.
