Best-balanced 3.5 classes...

[Lert, A.]

EDIT:
Really? The Psion seems to be about the same power level as the Sorcerer. Not one of The Big 5, certainly, but still full-caster-style game-breaking.

I find the powers to be a bit more balanced than the Sorcerer's arcane spells, which brings the class down a bit.

There are always the insane power builds, but it is a lot harder - in my experience - to make such a build.

The biggest problem I've seen that people have with the Psion is the nova aspect that earlier level characters have, where they can just blow all their PP in a single encounter and then have nothing left for the rest of the day. Depending on the DM and his style of play - especially with the D&D "I stop and recharge my spells" mentality that often prevails - this can be a problem, but the same could be said for a lot of classes.

EDIT: I see the Binder mentioned. Love using that class and feel disappointed that I didn't mention it earlier.