Beyond the battery question, the basis for using iPads is as simple as anything else involving the FAA. Tongue in cheek, firmly planted. VFR Part 91 flyers (except 91F & 91K) can use them freely. Conditions & stipulations multiply quickly as you move up the food chain into 121 & 135 operations.
Admittedly tedious to read, AC 120-76B provides some pretty good insight into the many ways that EFBs can be useful, and realistically, several potential gotchas that are helpful to keep in mind as our dependency grows. Next time you're killing time in the crew lounge, give the AC a quick read-through. You probably already have company or personal guidelines to follow, but familiarity with the AC could make you a better conversationalist if the FAA knocks on your cockpit window for a friendly chat.