They are definitely to be labeled as 'problematic'.
While it's certainly possible to have a campaign with psionics in it, where the classes are more or less balanced (like the level of balance the PHB sets), it *requires* the DM to take special steps towards this, whereas normally this is not the case (or not nearly to the same extent).
The biggest problem with the Psion class, which I consider the most unbalanced base class printed by WotC to date, is that they possess pretty much all the advantages of Sorcerers with (pretty much) none of the disadvantages (they are modelled after the Wizard, but still get full (and much more flexible) spontaneous casting).
And Psions have powers to gain extra actions, which is a huge design flaw in the psionics system (something that got removed (except for some weird creatures) specifically in the transition from 3.0 to 3.5, because it was too powerful).
Psions do have some psionics-specific disadvantages as well (a big point is the lack of party-friendly powers, and the general amount of available powers, although there are certainly more than enough out there by now with all the additions available (if you include 3rd-party products)), but they simply are not enough to balance their huge advantages.
Some people say, that Psions have the 'disadvantage' of being able to squeeze out and thus expend their PP quickly... now, I don't know... how can this be a disadvantage? Being able to generate more effect in shorter time is bad? That's kinda like saying a 9th-level spell is worse than a 1st-level spell, since it costs more (effectively) to cast it.
If you cannot restrain yourself, that's your fault, not the one of the class.
It's a big problem, though, if you can manifest so much in rapid succession, because that is what leads to the problem mentioned in the beginning, that Psions can only ever be remotely balanced when the DM *forces* the PCs through a plethora of encounters per day.
Magic has this same phenomenon at higher levels, but it's much more tame (and thus much better balanced).
Another point is, that the fluff to mechanics transition is questionable at best. Psions are the best nukers in the game. At least for me, that isn't quite the fluff I would expect from psionics. And all things considered, they are still much too similar to the arcane casters. Most of their powers are just the same or slightly altered versions of existing spells (often better, sometimes worse). There are a few unique things, but too little to make it worth bothering to add psionics to a campaign (and suffer from the campaign-altering effects, psionics inevitably carry with them) IMHO.
If you really like the psionics from a flavor point of view (or totally hate the 'vancian magic', or have munchkinny tendencies

), it might be worth a look, otherwise I don't think so.
Bye
Thanee