Hi,
A direct comparison between
Micro: http://arduino.cc/en/Main/ArduinoBoardMicro
and
Nano: http://arduino.cc/en/Main/ArduinoBoardNano
makes me think that the only Micro advantage is the ability to act as a USB Human Input Device. If you want that, it makes sense. But well-done Nano compatibles cost a little less than the Micro.
The USB onboard the Micro (and the UNO) is both an advantage and a liability. I've seen several UNOs that have lost their USB firmware and the end users do not have the ability or know-how to reload the USB interface. I've only seen one FTDI chip go bad in over 3000 328 based boards.
Essentially the Micro is a physically small Leonardo and the Nano is a physically small Duemilanoave.
But I think it's good thinking and good design work, and if some really cool projects use it and make it's design choices work well, it's a winner.