I've read up on the cordless impact thread, but am still torn between these two guns.
The IR will cost about $10-$30 less, weighs about 2lbs less, has lithium-ion batteries and is slightly smaller than the Snap-On.
Judging by 96sleepers tq. testing, both the guns put out about the same in reverse.
I might have the chance to get the Snap-On for $480 off a truck and the IR for $470 shipped w/2 batteries as well.
Here's a video of both of them in action together...

not sure how accurate it is, but ya. Click on "see it in action" and then the "showdown"
http://www.motherofallcordlessimpactwrenches.com/
The duties would mainly consist of exhaust, suspension and brake work, so those could be the most stubborn bolts.