The difference in using your phone with taped over cam, costing you $5 bucks, is nothing compared to buying a dedicated device in the $100+ range, that is only slightly better for certain types of short-range radiation like (alpha and low-energy beta). In addition the cam CMOS sensors doesn't saturate under high doses, like an ionization based device, and thus provides better accuracy!
The sensitivity depends on the physics of your mobile camera, where most phone cams start picking up radiation at about 10 µSv/h. For the iPad2 it picks up radiation already at the 1 µSv/h level.
So unless you are making high-precision, low-energy or background radiation measurements (like looking for Radon in houses), this app is worth every penny. In addition it is one of the best supported apps out there, with a solid scientific base.
Hack-A-Day - presentation and review
Developer Web Site
I especially like the radioactive Bavarian mushrooms YouTube video!