Tried this out yesterday (called it by a different name in the workout), and I liked it. Worked the muscles a little differently than normal, and seems relatively low impact in comparison to a squat (trying to save my poor knees).
It focuses on the glutes and the hamstrings, but expect some work out of your core (especially if you've got a weak lower back/bad hips). This exercise reminds me of a variation I used to do with a Swiss ball, but seeing as I don't have one, this will do for now.