This things rocks my socks. OK, so it costs as much as all four tires on your car, but if you're ever on the side of a stage road in the rain with a flat or decide to psyche out your competition with a last moment tire change, you'll appreciate the way it puts the power of an impact in a portable form. Uh, I guess you could use it in your driveway too. A nice gift for yourself, you deserve it!
I got one of the super cheap 12volt powered ones a while back, it was good for breaking lugs loose, not much faster than using the tire iron, I never used it to tighten lugs, cuz i'm a ludite and like to tighten my lugs by hand.
How does the snap-on one work? does it spool up and when you push the chuck in a bit it goes k-thud?
It works the same as a regular impact. k-thud, k-thud. It has a good rate of speed, not as fast as a proper air wrench, but waaaaaaaay faster than by hand. We keep an extra battery pack handy, but one pack is usually good for a day of racing (Usually 2-3 tire changes in service).
one of my mechanic friends has one. definitely worth it. it works just as fast as a regular impact (and sounds like it), but it's less powerful. 160ft lb of torque compared to the 400 my 1/2" air impact does.
but when you're at the track it's freakin awesome to do a tire change in a minute- by yourself. me want.
My dad is a tool salesman so I have a lot of experience with many different tools and brands. Snap On had some quality control issues with that series for a while. I believe they got it sorted out but just pay attention to how it is working and if it acts up get it back to Snap On
on my wish list too 
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