I've been doing it about 3x per week now for a couple of months now, and I love it.
It is a deceptive workout... the first time I did it I didn't really feel like I was getting much of a workout, but the next day... boy did I feel it! The lack of sweat can fool you.
I don't know a whole lot about it, other then what we do at our class. After warming up, we put on those foam belts with our foam dumbells, and go into the deep end for the aerobics portion. This involves doing moves like "x-country skiing", bicycles, and jumping jacks (arm movements are done underwater using the dumbells). The resistance created by the water, the currents and the dumbells makes it quite challenging. I believe you can also get weighted shoes to make the work out even more intense.
The best part about it for me is that even though it's a good efficient workout, it doesn't exhaust me, so on most days I'm able to get in a second full workout in (I'll either do a video earlier in the day, swim some laps or just use the stationary bike at the pool for an extra 30 minutes.) I'm still kind of new to exercising, that I know if I tried to do 2 full "land" workouts in one day, I'd wear myself out very fast.
Rebecca