I have both the Milwaukee M12s and the similar Bosch units. They are great from a portability and versatility standpoint. You really can't go wrong with either brand, but right now the Bosch's are better.
Bosch introduced them to the market in about 2005 as 10.8 volts, then about eighteen months later Milwaukee came out with their M12's (which are actually 10.8 volts!) and the M12's at the time bested Bosch in several features, as well as power and speed.
Then in the fall of 2010, Bosch upgraded theirs (and now called them 12 volts, even though they are still 10.8 volts). The newest generation Boschs are more compact, much faster and more powerful than the Milwaukees. I expect Milwaukee will introduce upgraded models sometime soon and will up the ante again.
Makita and Dewault also brought out something similar last fall, so there are several choices now. One thing you'll want to take a look at is the other tools that share the same batteries - Both Milwaukee and Bosch have a very extensive line of companion tools, and there are quite a few tools that are unique to one or the other. That might tip your decision.