I wondered this for quite a while, too. Actually, when I first saw it, I wondered "why bother?" because I think I had all of the others. Well, I got into a phase where I really liked circuits, so I did recently acquire it. I've only done one workout on it so far, but it was really fun. It's just different than doing any one of them itself, obviously -- it just gives you more options. I'm not sure I helped you at all, but I think it's "worth it" if you like circuit-style workouts.
-Beth