I had to jazz up psions to make them work in 3.0. In XPH, I can run them "out of the box" so to speak. If you are relying on high-con savant builds, though, you are in for a shock. (For that matter, almost every psion build other than shapers could be radically altered.)
Psionic combat is gone; all modes have become powers. If you dig psionic combat, get Hyperconscious by Malhavoc.
Psychic warriors also come into their own. They drop their vestiges of fighter-ness and they now have their own feel.
I'm not too hot on the soulknife, and I do have a few compatability issues. Some 3e material was built on the one-discipline-per-ability score correspondance, which has been eliminated. While this creates the biggest compatability problem, it gets rid of some balance problems, so is probably worth it in the long run. I can't get over Githyanki now being bad Psychic Warriors, though. Hmmph.