Who have you been talking to? Complete Psion is better than Complete Divine, but that's not hard. If you like psi, then the CP is probably worth it. But only because it's the only support psi is getting. If you don't like psi, then this book won't make it any more palatable. It doesn't suck, but it's definitely at the lower end of acceptable quality.
The Ardent is pretty cool (as Psion said, it makes the godless cleric a non-issue), and there are a few feats and powers that are interesting. Entirely too much of the book, though, is taken up by racial (illithid, dromite, etc.) feats, racial levels, and such things. There are also large amounts of material that has no "wow" factor, despite being servicible (the entire PrC chapter, for instance).
As others have said, it serves to expand the XPH the same way the rest of the "Complete" line expands on the PHB. Of course, you can still use the XPH info from the SRD.