I must say I'm very sceptical about any machines for abdominal and trunk extensor muscles. A common problem is that they put you in a position that makes the hip flexors work at least as much as the targeted muscles, which is implicated in many people's low back pain. You need to make sure the movement in coming from the trunk, rather than the hip. This particularly applies to sit-ups.

I much prefer static exercises for the trunk, such as The Plank and other variations on it.