Note that this answer addresses only whether or not 3.5’s psionics subsystem is imbalanced in comparison to similar options. It does not address whether or not a given DM should allow the material, only whether or not a DM need be concerned about its balance when deciding whether or not to allow it. On a personal note, I heartily recommend that any and all DMs allow it, since it’s great material, but that is not the purpose of this answer.
Psionics is far better-designed and tightly-balanced than 3.5 Vancian spellcasters
The Expanded Psionics Handbook is one of the best-written books and best-designed subsystems in all of 3.5. The Player’s Handbook is one of the worst on both those scores, as it turns out – they made a lot of mistakes early on in the game.
For the most part, it makes the most sense to compare the psion, psychic warrior, and wilder to the cleric, druid, sorcerer, and wizard. The soulknife does not really use psionics, and is comparable to the monk (and like the monk, is a stunning failure of design).
Some general observations on the differences between psionics and vancian spellcasting:
Psionics makes you pay for scaling
Spells automatically improve as your caster level goes up: fireball’s damage, for example, is 1d6 per level. When caster level goes up, so does the fireball’s damage.
Not so with psionics. Energy ball makes you pay Power Points to improve the damage, gaining 1d6 damage per Power Point. Since the Power Points you can spend are capped at your manifester level, this is equivalent to saying that fireball deals 5d6 damage if cast from a 3rd-level spell slot, 7d6 if cast from a 4th-level spell slot, and so on.
This Augmentation mechanic does occasionally offer things that spells would not automatically get – psionic charm can be Augmented from a charm person analogue all the way up to a charm monster analogue with enough Power Points. This does make some psionic powers a little more flexible than the equivalent spells. Since you always have to pay for it, however, this does not especially add to the power available; it’s just different.
All manifesters are spontanteous
Prepared spellcasting is a huge advantage. It means you can tailor your load-out every day, and no problem takes more than 24 hours to have the perfect tool. It makes it trivial to try different things out, to meet requirements for magic items, feats, and prestige classes, and so on.
But all manifesters are spontaneous, à la the sorcerer. Thus, they have an extremely limited (particularly in the case of psychic warrior and wilder) selection of powers. The flexibility of psionic powers due to Augmentation makes this a little better than it would otherwise be, but then the sorcerer has far more spells known than even the psion.
Metapsionics has a harsh limit that is absent on metamagic
It is very difficult to apply multiple metapsionic feats to a single power manifestation, since you must expend your Psionic Focus in order to use a metapsionic feat. Once expended, you do not have it to expend for a second metapsionic feat. There are a few, fairly-expensive ways to get more than one Psionic Focus, so you can expend two or three on one manifestation, but these are limited, expensive tricks. You might get to do that a once a battle, if you build for it. You will never be able to stack a suite of metamagic feats on every power the way some spellcasters can with their spells.
Thus, psionics is “balanced”
By the definition supplied by the question, psionics is very distinctly balanced:
- An option is overpowered if, when presented as a choice, it will always be chosen by members of a group.
No. Spellcasting has more options, more of which are overpowered, and has several areas of expertise (e.g. necromancy) that psionics lacks entirely. Cleric or druid can be far more potent “magic warriors” than the psychic warrior, and sorcerer and wizard are much more potent than psion or (especially) wilder.
- An option is balanced if, when presented as a choice, if will be chosen sometimes, due to its ability to fulfil requirements.
This much is true. Psionics does have a few fortés; it tends to be better at mental manipulation (shocking, no?), teleportation (especially short-range, tactical teleportation), and time-manipulation (at very-high levels). The psychic warrior more gracefully melds powers and martial ability than do the cleric or druid (even though they are much more powerful).
- An option is underpowered if, when presented as a choice, it will always be ignored by the group.
The psion has a few options that aren’t available to vancian spellcasters, as discussed above.
The wilder might be this, but because of the psion. There is almost no reason to choose the wilder over the psion, since the wilder gets only some of the psion’s powers, not all, and Wild Surge is mostly too dangerous to actually use, and the Overchannel feat is conceptually similar but far more bearable. For the flavor, a psion with the Overchannel feat is far superior and nigh-identical in the narrative.
And the psychic warrior, because of his psionic powers, is much more versatile and capable than mundane warriors.
Which brings us to my salient point:
3.5 is not a balanced system, particularly its core
The magic classes absolutely dominate the mundane classes.
Even with every supplement in the game available, the cleric, druid, and wizard are three of the five most powerful classes in the game (along with the archivist, which is just a divine wizard using the cleric spell list anyway, and the artificer, which makes magic items primarily from the cleric, druid, and sor/wiz spell lists).
Even with all the supplement support they get, the fighter, monk, paladin, and ranger are generally four of the weakest classes in the game; they do share that distinction with more classes since most mundane classes land in the same level.
No other book in the system has such an extreme dichotomy, with all but 4 classes being in the absolute best or absolute worst tiers of power available.
But Expanded Psionics Handbook is one of the best-balanced books
By contrast, Expanded Psionics Handbook has three classes that are all fairly close in power to one another. The psion is much better than the wilder, which is probably a bit better than psychic warrior, but all three are weaker than cleric, druid, sorcerer, or wizard, but also all three are stronger than barbarian, fighter, monk, paladin, ranger, or rogue.
Thus, the psion, psychic warrior, and wilder are
Relatively tight in power amongst themselves.
Fit right in the middle of the power levels found in Player’s Handbook.
The soulknife, I have left out. It does not use psionics at all (merely gaining a single pair of Power Points from its Wild Talent bonus feat), and it is miserably poor, quite possibly weaker even than the monk. Psychic Strike is unable to damage an enormous number of creatures, and is severely limited by its action requirements, making it all-but-impossible to land more than once per turn. Beyond that, its class features amount to “I have a magic sword,” which is a statement that can be said by just about anyone in the game who wants to. The soulknife is a black mark on an otherwise outstanding book. Only Tome of Battle has a higher proportion of balanced classes (since all three base classes are very competitive to one another, and also fit neatly in the middle of 3.5 power levels).
Also, this is 3.5: everything is exploitable.
There are exploits for psionics. At the high end (and abusing the far less-well-designed material in Complete Psionics), some of the exploits are comparable to the worst that spellcasting has available. Psychic reformation and quintessence show up in a fair few theoretical-optimization exercises for their unique properties (even though neither one is particularly overpowered in most practical situations). Synchronicity, Metapsionic Power, and Linked Power from Complete Psionics are all pretty much just broken. But then, so are contingency, gate (has no psionic equivalent), shapechange (psionic equivalent costs 200 XP every time it’s used), and time stop.
So I will not claim that psionics is free of exploit; nothing in 3.5 is. A kobold ex-paladin 1 can have every feature of every race, class, or monster he likes, with scores in every single thing as high as he likes. This is not a balanced game.