It's a good game, though I am not convinced that PEG will be supporting it heavily for much longer (I think their focus will move to Savage Worlds ... after all, Deadlands is getting pretty long in the tooth at this stage).
If you plan to play the original rules (and Deadlands d20 is at best a half-hearted product, so I suggest you do) then you'll really need the main book, Quick & the Dead, the 1877 update, and probably a couple of the splatbooks if you plan to use the arcane classes they - especially the Huckster - need the amended rules and extra abilities those books offer in order to be properly playable, IMO).
System-wise, Deadlands takes a little while to get used to (it is quite eccentric), but once you have a handle on it, I think you'll find that it works well, and injects a lot of flavour into the game (it's one of the few games I have seen where the mechanics have been designed as much to help set the mood as to determine the results of actions).