I have had this pilates dvd for ages. It is very core focused in my opinion, and challenging and easy to follow, too.
I don't do it as often as I should because I prefer other activities, but I like the production quality of this, the price $8.99, and the intensity. My husband, who's at a healthy weight and jogs, tried it once and found it very difficult. I think my husband wished he could have blamed his difficulties on being a guy, but the instructor on the video is a guy--lol.
Edited to add: the guy focuses a lot on breathing, which is why I'm recommending it even though you're asking about yoga. If I breath correctly, it really makes a difference in whether I can do the moves and for how long I can do them. And of course, because of the twisting and sometimes inverted positions, I get some stagnant mucus moving.
http://www.amazon.com/Rael-Pilates-...8&qid=1345220763&sr=1-44&keywords=pilates+dvd