Ive used the 18v Snap On quite a bit and they're great.
Personally, I have both the Milwaukee M18 1/2" Impact and the 1/4" Hex impact. The 1/2 is no joke, it puts out 450 ft/lbs of torque. These are the newer 18 volt lithium ion tools, I can't comment on the 14.4 volt that kennith linked to above.