Chris is right, to run air tools for any length of time you will need a good air tank with a 100% duty cycle compressor. Still with that, even a good compressor is going to have a hard time keeping up with some air tools. If you looking at airing tires, kids floaties, and other things that don't require a large PSI and CFM output, then you would be good with on board air.
If you really need to go somewhere and run your impact gun allot, I would look more toward a CO2 tank. They are more versatile in the sense they can be transported from one rig to the next. If one of your friends needs air, but is in a sketchy sitch, you can just bring the tank, instead of trying to snake your rig up to him. We've sold them to roofers and builders... They prefer a 5 gal CO2 on their nailer, than dragging a hose everywhere. The only major draw back is you have a limited amount of air in them, as where the onboard air can eventually recover.
Let me know what your main goal is, and we can try and work you into the best fit.
Thanks
Johnny Kurz
Tech/Sales Associate
Wheeler's Offroad Inc.
(541)474-2879
Johnny@wheelersoffroad.com