I bought these at Harbor Freight a few years ago for about $160 with coupons.
The car was pretty easy to move around until I put the engine and transmission in. Now it takes a little tugging and shoving but they work. The stuff about rolling a car around with a the touch of a finger is pure BS.
These things are great for pulling wheels off and reinstalling. Just jack the car up enough to unload the wheel, spin off the knockoff and roll it out from under the car. Reinstallation is just as easy because everything is lined up already.