I did bikram regularly for about a year. It's really awesome. Here are my thoughts:
You're in a hot room, they work the same series of poses every time. Which I loved, because you can really improve with repeated practice. You need to absolutely come to that class hydrated (I mean HYDRATED) b/c you'll sweat a lot. You need to drink a few liters of water for the rest of the day or you will get a headache.
This exercise is aerobic to the max. Imagine doing each pose once for :60 and then once for :30. In a sauna. It's regemented, but the series does flow after you've done it a few times.
The cool thing - you really do detox a lot through sweat. I never felt better than when I was doing bikram every day. You'll lose weight and tone up over 1-1.5 months - you'll notice a difference even going three times a week. It's very intense. VERY intense. I liked doing it in the morning, but you're really zapped out afterwards. Doing it in the evening is good if you do it early enough. Some people can't sleep after doing vigorous exercise.
It's pretty expensive (at least in NYC, and compared to other yoga methods). This is to pay for the specialized studio that's heated and well ventilated at the same time.
Now: The one thing - the bikram instructors are good, but they only have to do a very specialized bikram training course of 3 months. Whereas non heated yoga teachers usually learn for a lot longer about the systems of the body. I ended up moving over to anusara yoga, which is non-heated and the teachers teach intelligent body alignment. However, Anusara never kicked my butt like bikram!