The Game Boy Micro is stupid and anyone who denies it is a biased fanboy (as opposed to the more objective fanboy) or a Nintendo employee. The first problem a consumer will notice is the systems price. A hundred bucks for something you probably already have a different version of?
The next obvious problem is the tiny screen. I already have trouble playing action games on my GBA-SP because the screen isn’t very big. The tiny screen is a part of the overall tiny system. This is the major selling point but it’s also one of the best reasons not to buy one. The GBA-SP already hurts my hands. If the SP is designed for Japanese fingers then the Micro is design for Japanese ants’ fingers.
Then there is the competition. If you have the money, just get an SP. It plays more games (original GB and GB Color games, the Micro only plays Advance games), will be easier on your eyes and won’t cause the carpel tunnel to flare up as badly. → Hell is other gamers.