^I have a friend who does Bikram Yoga 9 times a week (once every weekday and twice on Saturdays and Sundays), and she absolutely swears by it - in fact, I think she's planning on becoming an instructor. Some other friends have gone with her once or twice and seem to think it's very intense with the heat. But they all agree it's an effective workout and you sweat off a lot of calories.
Last year I was really into Bikam yoga. It does wonders. I went twice a week. My muscles became very lean and toned because you can get the extra stretch in when your muscles are nice and warm. Make sure you pre-hydrate before going to classes (about 24oz of h2o) and to rehydrate after the class. I also found it a little helpful to have some protein/fruit about two hours before the class (string cheese or half of a banana worked for me). I am trying to get back into it, I moved last year and haven't been able to find a studio that works with my work schedule : (
Thanks for the responses everyone! I think I'll try to make it to some classes. Currently I have a regular iyengar yoga practice, but the Bikram studio near my house ALWAYS has a class on. It's so convenient for me!