For power/gearing, it should be equivalent to swapping on really small tires. This probably limits your top speed even if the track design doesn't, but who cares? Whatever you plan to run these in, you're gonna want *equivalent* low gearing anyway. The effective gearing should be the same as running tires the diameter of that top wheel in the track.
From everything I've read, they only bolt on to your wheel studs, but I wonder if there isn't something else to limit their movement. I mean, you would want them to rotate a bit to match the terrain, but what keeps one from turning forward or back and chewing up the fenders?
I wonder if they are street legal since they are rubber? Sure would hate to see track-wear end the life of these, since they cost so much.