I just got The Firm's Cardio Dance Club. I love it! It's pretty low impact, easy enough to do, and you just have to remember to keep your tummy tight and butt tucked, that's a HUGE way to make sure you work on those muscles and protect your back from injury.
I like it because it has 3 separate 20 minute workouts, so you can change it up. It's low impact. And this is a personal preference, but it gets right into the moves instead of that annoying "this is how each move is done" intro. I don't like those because they bore me and then I'm not motivated to work out anymore.
Did I ramble enough? lol