I'm with Seule. Psions aren't underpowered; I have a Psion and a Sorcerer (they actually started as two versions of the same character, depending on whether the DM let me use Psionics).
Multiclass Psions can use armor without penalty (mine's an Aristocrat/Shaper/(custom prestige class)). They have more flexibility in casting (my Sorcerer often runs out of level 3 slots while still having almost all of his level 2 slots remaining). They can benefit from high stats other than the useless CHA. Certain spells, like divinations or mind control, come at lower levels than for a Sorcerer. Their class skill list is much better, no matter which discipline you pick. They can still cast while being Held, Silenced, Polymorphed, etc. They can use Psionic Attack modes to stun people for cheap. They have no need to carry around a bag of bat guano (material components). They have access to a lot of nice Feats requiring a power reserve (like Inertial Armor or Speed of Thought).
On the downside, they get about 33% less spells per day total once you count in stat bonuses, they suck at damage spells, and once they get to high levels they're really limited to their prime discipline. The d20 DC thing is both good and bad. A Familiar has more uses than a Psicrystal, although it's not as happy to stay in your backpack.
I think they're balanced nicely as is, maybe a bit TOO strong.