My tandem stoker goes to spin classes to keep fit for our rides when the weather warms up (she doesn't like riding in cold weather or rain, and she's not even from California!). She says the instructors (all female) are usually triathletes, so the classes can get pretty intense, but she goes at her own pace. She does try to increase her spin rate, though, which is a bonus as she's getting more used to spinning at my tempo on the tandem.
I once went to a spin class. It only went about 40 minutes, but by the end I definitely had the biggest pool of sweat underneath my bike. I think that the fitter you are, the easier it is to sweat because your body is just more efficient. I was impressed that all the bikes had SPD-compatible pedals on them, the kind that you can use as platform pedals or clip-ins. I used my mountain bike shoes. But overall, I figured I'd rather be out on a real bike.

Luis