I've got a bad habit of picking up spare styrofoam sheets on the side of the road that have blown off trucks or building sites.
They aren't cheap off the shelf, but off the ground, free. They are comfortable, provide excellent insulation from the ground, and smooth out any and all gravel/bolts. They are also lower profile than the car dollys.
Where your magical carpet wins is in the stowability and movement department.
After using the stryofoam sheets, I see the possibilties in the carpet thingee. I am tired of dolly wheels getting stuck on random bolts underneath me. My 10mm sockets have no place to hide.
Only disadvantage to the carpet/styrofoam I see it the ability to move long distances sideways when under the car when you need a better angle on something.
as far as the carpet, I am a believer. I might buy one.