I am most definitely pro-psionics, but not fanatically so.
In the polls, I've recently been voting to keep the psion and psychic warrior around because I honestly like their flavor, mechanics, and roles better than the other classes that have survived this far. For most of the polls I just voted against classes I really disliked, whether from thinking that they were unbalanced, or thinking they were redundant, or thinking that they made some of the core 11 redundant and worthless, or thinking they were just stupid concepts.
To my knowledge, most folks who dislike psionics in D&D have one or more of the following reasons:
1) they think it's too much like sci-fi, or that its flavor is to pseudo-scientific;
2) they don't think it has any place in a medieval fantasy RPG, because it has no significant basis in European myth and legend, for the same reasons such folks dislike oriental classes like the monk;
3) they believe that the psionics rules are too broken and unbalanced, to the point of not even giving them a fair chance in the first place, in some cases just because of bad experiences with previous editions of the rules;
4) they don't want to learn a new sub-system, no matter how much it is or isn't like the core spellcasting rules, and they think it's just too complicated on top of already having to remember the combat rules and spellcasting rules;
and/or 5) they don't like the flavor of psionics as it stands right now, and can't be bothered to just accept an alternate description of the powers and their origins, like one of the several alternate flavors of psionics that have been used by lots of groups already, and even discussed on the forums occasionally.
In Eberron psionics is just a different power than magic, but linked with Dal Quor, the realm of dreams, and is intentionally left as an optional part of Eberron campaigns since it is only significantly used by Kalashtar and the Inspired, on a separate continent from the main campaign regions, so folks can use or ignore it as they please in that setting. In the Tablelands of the Dark Sun setting (errg, brain fart, can't remember what the plane is called), psionics is just the dominant supernatural power around because magic is either of the arcane sort (hoarded by the sorcerer-kings and some comparatively weaker groups, with the hindrance of being either preservers with weaker magic or defilers who have to destroy nearby plantlife and fertile soil in order to cast strong spells), or the divine sort (restricted to some templars who draw power from the sorcerer-kings IIRC, and elemental clerics, who are few because they have to get the support of an archomental or whatever). In Faerun/Abeir-Toril, psionics is only possessed by some non-natives and their descendants, such as mind flayers, yuan-ti, and a few planewalkers who've settled on Toril, so it's practically inconsequential. In Monte Cook's Arcana Unearthed, psionics is just a spell descriptor for a handful of spells, that involve the use of telepathy, telekinesis, or precognition, to some extent.
Generally, people who dislike psionics just don't want to bother with any of the alternate descriptions for them because those descriptions aren't "canon" to the main D&D rules, or they just don't want to bother reading enough of the main psionics rulebook to avoid any rules-violations by players, or they just don't want to add a few houserules to their notes to keep psionic characters properly in check. Of course they wouldn't need to, if WotC didn't rush out every book and allow every designer to miss the balance problems with a few powers each time. The psionics system can easily be re-described by DMs as a form of mind-magic, or a greater and more enlightened form of ki use, or a primal connection to the minds of gods, or a strange connection to certain forces of the planes/multiverse, or an alternate magic system plain and simple, or whatever. Many folks just don't want to bother since they want Wizards of the Coast to psychically predict and tailor every single book and game element to their invidual tastes, everyone else be damned, just to save them a bit of time.
I liked 2E psionics and 3E psionics, but while I was less pleased with 3E psionics, I was very displeased with 3.5 psionics. 3.5 turned the focus of psionics ever-more into the realm of science-fictiony terms and powers, and added a godawful-stupid 'crystalpunk' theme (as I understand people have been calling it since the XPH came out). Before 3E, there was no excessive focus in the psionics rules on ectoplasm or crystal stuff; 3.0 psionics had a bit of focus on crystal and a moderate focus on ectoplasm, but not too bad, until 3.5 went whole-hog with both stupid concepts and ruined the already-fragile image of psionics. It was only slightly scientific-like in 2E, but 3E/3.5E added more pseudo-science terms to the mix rather than sticking with 2E's more eastern flavor.
I would've preferred it if 3E psionics was just a simplified and finely-tuned version of 2E psionics, which weren't all that bad aside from a few complicated or vague powers. Most psionic powers in 2E were very, very weak and limited in scope; devotions were like 1st-level spells or occasionally 2nd-level spells, and sciences were like 4th, 5th, or 6th-level spells, and the more powerful sciences or devotions had prerequisites too, which generally consisted of several weak powers that needed to be learned first. A revised version of 2E psionics would have been much more appropriate to D&D and less complicated, as well as having none of the ecto-bloat and crystal-fetish that 3.5E psionics retardedly includes.
I know full well that the psionics system has been unbalanced in spots throughout the past 3-1/2 editions, but it's not like it's been all-around overpowered the entire time. There have just been a few broken powers in each edition, that some fool designer failed to realize he'd/she'd overpowered.
I've never seen a psionic character outshine the rest of the party, as opposed to the several times I've seen melee brutes, absurdly-focused archers, and trigger-happy arcane twinks totally outdo the other PCs. As long as the DM knows what he's dealing with, a munchkin can't pull the wool over his eyes any easier with psionics than with anything else in D&D.
But, for DMs who don't bother to read the psionics rules to any reasonable extent, it's much easier for a munchkin to ruin their game with twinked-out psionicists that blithely ignore some of the rules that the DM hasn't bothered to read. Or that just use certain combinations of powers that any knowledgeable, sane DM would have outlawed (though allowing them individually; certain combinations just get wonky).
For some reason, the designers never bother to put as much effort into balancing the psionic classes as they do with the spellcasters. So a handful of broken powers, or broken combinations of powers, slip through every time due to lax authors.