Metamorphosis has most of the potential for abuse that polymorph does, IIRC. It's major limiting factor is that it is self only and only a Egoist gets it naturally on his power list. All others have to spend a feat to get it.

However, this means it is limited to the manifesting psionic classes, most of which don't have the hp and BAB to really get the most milage out of it. The one that does (psychic warrior) can't take it until 13th level and psywarriors are outragously capable buff-monkeys anyway.