I am in the same boat. My much loved Makita 12v batteries (4 count) are all but gone, and I am weighing the Milwaukee M-12 system vs 2 or 4 new batteries.

Edit to add, the M-12 system includes a reciprocating saw. I had need for one in the field recently, so it is an interesting advantage. Kind of leaning toward 2 new Makita batteries and then the M-12 if I need the saw, and then building up the M-12 system.

For what it's worth:
http://www.youtube.com/watch?v=_KcRBnO7wVA
Ag.