I haven't tried one, but I watched a demonstration closely a while back. I noticed that all of the movement was in the hips, where the body opened and closed like scissors. I didn't see the abs being exercised at all, like you would if you were doing crunches. It didn't look like it would work at all
I think most of these gadgets are just expensive gimmicks that can't do anymore than a traditional crunch. ACE Fitness did a study a couple of years ago to see if the various ab machines could beat a crunch, but the results were not good. Remember the ever-popular Ab Roller? It turned out to be no more effective than a crunch. The Ab Rocker was LESS effective.
However, doing crunches on a stability ball appears to be more effective, according to ACE Fitness, and they are about 10 bucks at walmart.