Charles, have you used the CompM4? I have a couple of M2s, but they do suck batteries dry. I like the that they've finally switched to AA. I have too many optics that use weird batteries (like 'N' in the old Eotechs).
Quite a bit. I still haven't had to change the battery in the very first ones we got. I also greatly prefer the 2MOA dot vice the 4.
As far as the M2 in concerned, I still use one. It works. I leave it on the second or third highest setting and just replace the batteries about every six months.
When were talking versatility it is very hard to beat a 2 minute Aimpoint in conjunction with a magnifier. If most of my shots are under 300m and they are likely to come at any range within that 300m, the dot+magnifier works very well.
