Saw this on some show on Discovery - or maybe TLC - where people pitch products to potential backers. IRC they chose not to invest... but yes it looks really, really useful. http://www.humanhoist.com/
My garage floor doesn't stay clean enough for that, I'd crash it the first time I used it or I would manage to be under a vehicle with my arms above my head and somehow manage to hit the raise button/lever and pin myself under said vehicle.
How is this any different from using a normal creeper? The point is that it saves your knees and back from constantly bending over and laying down... the overall rolling-around isn't that much different.
I'd bet for the gain in comfort, you'd quickly change your behavior to make it work.
Or - build a tray into the side of the thing so you just lay the tools down right there...
On the show I saw, they took it to a garage and had several guys try it. They all said it was great and they would use it. I think the biggest problem was target cost. The holdback on investment wasn't b/c they didn't think it would work or would be a real hit, it was more a concern they had about limited market and not selling enough for them to make a lot of money as investors.