I think it might vary according to how advanced you are in yoga. You are definately burning calories doing it cause even playing cards burn calories. You are also building or firming up muscle. It takes me about 2 weeks to get back used to doing yoga when I get off track from it. I agree with the last poster though, it benefits you in other ways as well. It can help you deal with stress. They talk you in to a calm state of mind that you're suppose to focus only on what is going on with your body right then and there and not the world around you. I enjoy it mostly for that reason alone.
I have the AM and PM yoga and Rodney Yee does the AM. It isn't nearly as strenuous as the PM and you don't hold the poses quite as long but when you do, you can really feel the burn in the muscle. Oprah has also done a show where celebraties like Madonna claim that's all they do now for "workouts" and many models are into it as well. It will vary for each person as well as anything else will.