I also feel that Bikram is gimmicky. However, I'm careful not to knock it too much. I've practiced Ashtanga Vinyasa for about 10 years and was much opposed to the idea of copywriting yoga sequences the way that Bikram Choudry has done. The idea of heating the room is a marketing gimmick and is completely unnecessary. I oppose the use of a heated room for yoga because its just so wasteful of energy.
On the other hand, I have practiced heated yoga numerous times and it is a good introduction to yoga in that all the postures are fundamentally what a beginning yoga practitioner needs to learn. You can learn all these postures in any beginner class without the heat, and you'll still get the benefits. I believe Bikram is so popular with men because it gets you to sweat very fast. You won't necessarily get that from an unheated beginners class. But I assure you that practice Asthanga Yoga Primary Series and you will be sweating profusely by halfway through the series (which can take 1.5-2 hours to finish)!