The first Nintendo Game Boy micro reviews are available.

Kotaku got their hands on one and also IGN has had time playing with the new Nintendo Game Boy micro, which will be in stores in September.

Kotaku thinks $100 is a bit steep, but they are impressed with the Game Boy micro. (Photo from Kotaku).
The IGN reviewer shows that the Game Boy micro fits in his mouth. Either he has a big mouth or the micro is indeed small. IGN is impressed enough with the micro to prefer it more than the GBA SP. They also think the Game Boy micro should be cheaper.
See also the Nintendo Game Boy micro site.