I found the PrCs kinda mediocre, but Skill Tricks are for everybody. They're like cheap, little minifeats! It also has some fun spells that are good for punching up someone's spell list.
Another vote for CS. We have a paladin who just picked up the Grey Guard PrC, and it's a lot of fun. Plus, the feats for spellthieves (!) are great. And I'm making a Wu Jen character that's looking at the skill tricks and maybe even an initiative-based luck feat or seven.