I haven't done them since I started doing Cathe DVD's so I'm not sure how I'd like them now....but I used to find them challenging and enjoy them. Slim Series is a good set and Fast 10 is different and doesn't have as much variety as Slim Series. Now you've got me curious. I'll have to try them on my next recovery week and see how hard I think they are after having doing Cathe exclusively since February. I think those and the Kathy Smith Project You are the best of Beachbody.com (aside from Power Half Hour - that's a good one also).