Used them at the gym. They work but are not for the average infomercial watcher. You have to have strong abs already and have good form for most of the exercises.
The ones at the gym I go to also have foot straps on them, so you can crawl with them attached to your feet. you crawl in a plank position 10 yards forward , 10 yards back. Basically a walking plank position. arms walk and legs are on the wheel, core stays rigid(Power wheel crawls)
Other one is to roll out keeping back and arms completely straight. go from 90 to 180 degrees and back again. Power wheel rollouts is what the gym I go to calls it.
Also do pike ups using them