I don't like 3rd party stuff when it comes to PC development, and that's not because I feel that it's overpowered/low-quality/whatever, but because it tends to make the "field uneven" for the players. IMO, of course.
Player A, who has 100+ 3rd party books, can comb through all his books to come up with the super-cool (note: NOT super-powered, although that's possible) combination of feats and equipment or whatever for his character.
Meanwhile, Player B, who only has the core books and can't afford to buy a glut of 3rd party stuff, gets to use core stuff only. Maybe he would have liked to use some of the supplemental stuff, but he doesn't have access to it; and when Player A comes in with all this 3rd party niftiness maybe he feels slighted. Or not slighted, maybe, but... that his PC is inferior.
That's just my take on it. When it comes to DMing, I love 3rd party stuff. But for the players, they get Core and the 5 WotC Splats, and that's it. Give everyone the same resources to work with and nobody feels like someone else got a better deal 'cuz he owns more books.