Psionics suffers from the same problem as ToB, but to a lesser degree: A lack of optimization isn't as crippling as it is for core classes(minus Soulknife). For low-OP games, this *IS* a problem, since they have a hard time dialing back. Worse, psionics is *GREAT* at the one thing low-OP players want their casters to be able to do, blasting. Particularly devastating as a DM, you can easily wipe a party using SRD blasting powers.

On the other hand, in high-OP games, they start falling behind wizards, druids and clerics, who are more versatile.