I like the idea of a the controller in your left hand and your right on the mouse, but in the games I play, there tend to be a lot of buttons I press with my left hand and that setup doesn't allow for it. For example Salt, which has your typical first person shooter setup, I change weapons with the number keys, sprint with shift, jump with space, and use with E. If I want to use the mouse, and I do, the controller is useless to me.
I'd much rather have one those controllers with a thumbstick and a bunch of buttons for your left hand though I doubt I see myself buying one.