There's sort of this saying in the UX disciplines, that you never really know what you will learn when you do usability testing with real humans, but you will always learn something new every time you actually put the thing you're working on in someone else's hands. I can attest that yes, you do learn something every time, and you should try it. What I've found is that pretty much the same thing is true for doing varied device testing.

Read the whole post "On the value of testing on real devices"