I use mine several times a year. I love it, much safer than a hilift even on solid ground, and 10x better in snow or sand. It's designed to run off your exhaust, but if you can't get a good seal on the exhaust pipe you can also pump it up with a halfway decent 12v compressor. They're very sturdy - I wouldn't work under a truck supported by one unless I had to, but I would if I had to. Can't say the same for a hilift or bottle jack. For lifting the truck to change a flat or stack rocks/traction ramps, these are perfect.
It's basically leverage, each individual pulse of exhaust only expands the bag slightly so each one only needs to do a tiny amount of work. Even a little 4cyl motor will get a truck off the ground, as long as you don't have an exhaust leak.