The only problem I see with the Sure Grip Dollies is that you cannot move the front of the sled sideways.
I have a set of aluminum dollies I bought several years ago, that are still in service today. The wheels on mine rotate 360 degrees in any direction, which allows me to turn my sled around in the garage in a 10' diameter space. I can park parallel to a wall, then just push the sled tight up againt the wall. You could never do that with the Sure Grip Dollies. I can (and do) drive around on mine like they did in the Sure Grip Dollies video, except I cannot "really" steer, because the wheels rotate 360.
2004 Polaris Trail RMK 136
1997 Ski Doo Summit 500
2000 Arctic Cat ZL550 ESR (in NY)
2000 Arctic Cat 440 Panther (sold)
1980 Arctic Cat Jag 3000 F/C (in NY)
"If it ain't broke, take it apart and find out why!"