I just use one of those cardboard exposure computers. I have one in nearly every bag. They work well, cost next to nothing, are expandable (not much of a deal if you damage one), you can give them away, fast to operate, don't need booting, and so on.
I see no reason why the expo meter in the iPhone wouldn't be reliable. Cameras and meters have been relying on the CCD/CMOS chips for their exposure meters for a long time.