I really think the GBM is good for the casual gamer who wants to be able to play away from their TV. Let's face it, the PSP and DS are big, and you might not want to be carrying one around everywhere you go. The Micro seems truly small enough to go anywhere.
Of course, the downside to that is that because it's smaller, it's easier to misplace (ie., lose). Since the Game Boy market is predominantly children, this could be a problem.