I'm not familiar with it (but /agree above on there being no such thing as spot toning/fat loss) but it looks like there is an article right on the 3FC site that talks about the myths/facts relating to the ab rocker:
http://www.3fatchicks.com/myths-and-...the-ab-rocker/
Bottom line: "This machine is great for an abdominal workout, but will not do anything for any other part of the body. If this machine is the only exercise machine that is being used, there most likely will not be a whole lot of results seen. If there are results, they will be very slow and they will not be much."