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.