I took a cardio-kickboxing class here on a friend's recommendation, not really knowing what to expect. I walked away from the class (barely!) thoroughly exhausted, but impressed! The head instructor was enthusiastic and had the balanced motivational approach of a drill sergeant mixed with your favorite college professor. His routines were original and challenging, yet the sense of fun and enjoyment was always present throughout the class. The only knock I have is that I wish there were more water breaks, but maybe that's just because I was so rusty at the time!
Cons: SF parking always tough