I own the 3.0 Psionics book, but never really used it. It had some good ideas, but just too many problems with balance and mechanics.
I'm happy to report that the 3.5 version is FAR superior. Scalable powers, much better balance, many more choices. Many of the powers (and the way they are organized) have been rebuilt from the ground up. No more seperate attack and defense modes, for instance (I always felt they should simply be psionic powers, same as all the other powers, and now they are.) I'm excited about creating a psionic character now, and I want to use this book! (unlike the 3.0)
You know, I've been VERY hesitant to get into 3.5. I felt that 3.0 was good enough, and no way was I spending all that money on books that are essentially the same. But Mongoose's Pocket Players Handbook was cheap enough that I finally gave in and got it. While I still don't have the 3.5 versions of the DMs Guide and MM, I did buy Complete Warrior (despite having Sword and Fist) and now Psionics 3.5. I have to say that both of these books are vast improvements over their 3.0 counterparts, and if this significantly higher level of quality is maintained, they will eventually make a 3.5 convert out of me...