Most of the pilots I know who use an iPad as an EFB also carry paper charts as a backup. The real issue with the iPad in-flight, at least in a small airplane without A/C, is overheating and shutting down. I've seen this happen before and a backup iPad isn't much good in that situation. The fix is usually to remove any insulating case and direct a fresh-air vent at the device until it cools back down a bit.

Where the iOS devices shine is in the pre-flight weather briefing. Foreflight specifically is just an amazing tool for pre-flight planning. http://www.foreflight.com/mobile