iPad's are good for a lot of things, but heavy duty chess work is not necessarily one of them. There are number of chess playing programs available (I have stockfish on mine), and a few eBook apps devoted to chess (e+Chess Books another that I use).
However the most common question asked of me concerns PGN readers. As with most things in the iPad world, there are a number of apps you can purchase. As with most things in the chess world, most people don't want to do that. So the real question is: Is there are free pgn reader? The answer is yes, a couple, but the quality varies. But if you want a simple, free reader that works, then Chess Viewer from Everyman Chess is the one I use. While it is designed to enable you to view ebooks you purchase from them, it also reads standard pgn files.Loading them onto your iPad is fairly straightforward, as long as they are available on the web (you specify the web address of the file). After that you can select games to play through, and it even handles sub variations etc.
The only drawbacks for a players point of view are (a) no analysis engine and (b) no editing or data entry capabilities (it is just a game viewer).
So, this package is best for playing through games from tournaments etc. If you are looking for something that will do prep, post game analysis etc, then the first place to look is in your wallet.