I myself have always liked the concept of Psionics---I wasn't hardcore into gaming back when 2e was the new kid on the block, and never participated in 2e psionics, but I pretty universally hear that 2e psionics were VERY broken. Beyond never having had access to the psionics materials back then, there's not really any reason I didn't play one.
When the original 3.0 Psionics Handbook came out, I was thrilled---I personally like 3.0 MUCH better than 2e, and adding psionics to what I viewed as a "much better balanced and thought out" ruleset (don't bother to argue; I'm not saying that's how it is, just that it's what I think, and it's perfectly okay if it's *not* what you think...). Was it? Well, 3.0 Psionics were certainly cool in their day, IMO, and my group used them. To date, I've made a psion (who went very far, including into Metamind), and two psyhic warriors (one in a campaign that never got off the ground, and the other suffered an unlucky crit), and I enjoyed each of them.
Also, I did find that psionic combat pretty much never happened, as it added a new level of complexity to combat that most other players aren't really willing to sit through... I personally think the removal of psionic combat in the XPH was, overall, a good move---although I enjoyed the depth it added to psionic characters, it seemed a larger hassle than was worth, and was pretty nasty to those with the non-psionic buffer... 2d4 Wisdom damage---heck, even 1d4 Dex damage, for many characters---is pretty darn painful, but nothing compared to being stunned for 2d4 rounds... Or God forbid you choked on your save for Mind Blast---3 to 12 rounds? If the psion was smart, and brought along melee combatants, or has nasty area-effect damage powers himself, he sits back and toasts you. Clearly, there's a balance issue there.
I'll certainly never say that I'll never see a worthwhile psionic opponent, though... the aforementioned githyanki, for example, or if your DM allowed it (and you were playing the with "Psionics Are Different" rule), imagine the Psychic Warrior/Forsaker---You don't need magic... you've got psionics.
As for my homebrew world, I've tried to incorporate psionics directly into it, and *not* have psionics illegal (not everywhere, at least...

The city where psionics were legal but magic was not was very fun, if stressful, on my players---made them sweat a little... >
![Devious :] :]](http://www.enworld.org/forum/images/smilies/devious.png)
)... I like to think it worked out well. After all, not everywhere you go will be somewhere you like. ...Then again, I LOVE having options for my players.... I go to great lengths to make sure that there's PLENTY of different ways to take a character, to the point where a good 2/3rds of the work I do for my world will likely never be used by a PC... And that's okay. As long as they know it's there, then they're the ones that chose not to play it---I never said they couldn't.
Still, I think psionics seems to be "that addon" instead of a part of D&D... I suppose I can *sort* of see the arguement that psionics is a bit too sci-fi, rather than fantasy, but I personally believe that's just cuz things like Star Wars, a sci-fi fantasy (I'd just say sci-fi, but it's been argued that it's more mystical than that), essentially use it (Up the Walls? Telekinesis? Use the Force, Luke.), and have sort of "adopted" it as their own. On the other hand, I think the "psionics isn't present in classic fantasy" arguement is a bit odd... Arguably, *half* the things you find in the PHB, DMG, and MM don't appear in "classic fantasy".... It could be claimed that double-bladed swords shouldn't be included in the PHB, because they weren't actually used anywhere near what other weapons were, because of the realistic training required to wield one. And I've certainly never heard of a double-bladed sword in a tolkein book. And yet, in the PHB its entry sits. So why not psionics?
I never even remotely saw psionics as "steampunk". There's no gunpowder or technology to it, it's just the so-called "power of the mind". Making things happen with nothing more than the power of thought. Barring actual proof that psionic powers really exist (whether or not they do, I will naturally not get into), that sounds pretty "fantasy" to me.

...Still, the terminology doesn't need to define something... We call them dinosaurs. But some say way back when, they were called Dragon... Leviathan... Behemoth... Which is the "right" term? They weren't really called "dinosaurs" until the 1800's--does that mean "dinosaurs" (as in the monsters as listed in the MM) are "not fantasy"?
Honestly, I think people are just putting too much into what "psionics" includes today, and not really letting what they mean to the D&D game itself sink in.