The only time I use one on my Mazda 3 is when some idiots box me into a parallel parking spot.
Other than that, it is particularly useless in the winter since -due to its unprotected location and design- you'll drive 5 minutes and it'll be covered in muck.
I also wonder what this means for the likes of MB, BMW, Audi, etc. who have their backup cameras included in VERY expensive option packages. Will those option packages now become (HORROR!) standard?