Nice gimmick, but it didnt work very well for me. the watch doesnt render to the lighting conditions and just looked badly photoshopped on.
The most impressive service I came across was at a formex stand, they gave away these rubber wristbrands that have indexing points on them. Then you'd look at your wrist through the formex app wearing the band and it would live render the formex watch onto your wrist, you could move your wrist around and the watch would live render along with your wrist movement.
The formex feature gave you a decenly good idea of how the watch will size to your wrist. The mappinweb site is just guessing and might give you a really bad idea of sizing.