If you don't have much credits to spend, keep your old book.
Some of the background bits on the deities have been updated; most prestige classes are interesting (I'd like to play an elemental archon someday, just to hear the sycophantic fawning of my mephit underlings); it's good to have a Repose domain for clerics of Kelemvor or Jergal (much more appropriate than Death for them);
but that'll probably not be of critical use if you already own all three 2e books.
Furthermore, if like me you prefer to have no illos than poor illos, keep away from that book. I bought Deities & Demigods in order to have some well done divine pictures, F&P is just ughh 7 times out of 10.