I just wanted to share because these have been really helpful for me -- I found this reviewer on Amazon called "blueyed puella" (who knows, maybe she is a member here!) who has reviewed a lot of exercise DVDs in great detail. Some of her reviews are a few years old, but they still apply, obviously. You can check out her reviews at http://tinyurl.com/yqy434 , or you can just go to Amazon and search the People category with her screen name.
I have found her reviews to be fair and accurate (besides sharing her own opinion, she also tells you what's involved in each routine, how long it is, how much space it takes up, etc), and she's done homework for you on lots of different tapes.
Hope that is helpful to someone!
