It works well (but for many of us doing a rotation on another board, it often ended up being "Ab Skipper", LOL. I think the core results I got were as much from the other workouts as from this one.) After a couple of weeks, I started adding a weighted medecine ball to many of the moves (not much, just 2-3#, for variety) and/or doing the workout while seated on my rebounder (for a bit of a stability challenge that recruits the core more).
YOu will feel it in the hip flexors in the beginning, especially the first 2 or 3 exercises.
If you decide to buy it, DON'T buy it separately. It's included on each of the resistance workouts, so for the same price, you could get "Ab Rippr X" AND "Shoulders and Arms," for example.