As an avid roleplayer and owner of an iLiad (currently the e-book reader with the biggest screen), I'm afraid I can't really recommend it. Most PDF RPGs have a big enough border that you can increase the size of text on the screen so it's readable; or you can turn the page sideways and read half a page at a time. The main problem is the fact that turning a page takes a couple of seconds. So if you're reading an RPG page by page that's OK, but flicking through for a particular rule in the middle of a game is right out.
Roleplaying things you *can* do with an iLiad (but not other readers). If you've got a character sheet in PDF format, you can fill it in on screen with the scribble function, so you could keep your character info together. And it's a good scratch-pad for things like keeping track of combat. But the days of carrying a whole stack of rulebooks to your game in a nice small gadget have yet to come.