I rode a few hours on a borrowed Air Hawk last year - it was near the end of a long trip and my ass was really killing me. It helped quite a bit and I was reluctant to give it back to its owner when we parted ways. I've got the factory gel seat, which is great for day to day and shorter rides, but I plan on getting one before my next >4 hour ride.
As
@motolink points out, you don't want to blow it up all the way. The way it works is that it allows you to constantly shift position so all your weight is not resting on the same slab of butt meat for extended periods.