I have the "high volume" 12V unit from Harbor Freight (about $45 on sale). I was impressed with its volume (I routinely air down all 4 rear tires to 28 psi on the dragstrip, then need to inflate to 80 psi for the trip home). However, it tends to reach its thermal cutout point quickly, so it still takes a while in warm weather. So far it has still been the fastest option for me, other than firing up my Honda EU2000i and using a real air compressor.
I am about to add air bags to the truck, so I'll be using it much more for that function. I thought about rigging up a better heat sink and even a cooling fan for it (and mounting it to the truck semi-permanently), but that will have to wait for another rainy day.
Regards,
Michael Pliska