+1

I started spin classes about three weeks ago (two sessions a week) now that it's too cold/dark to get out for an after work ride. I'm still fitting in actual cycling on the weekends, but so far I've really enjoyed the cardio workout that I get in the spin class.

IMHO, how much you get out of it is heavily influenced by the instructor. My usual instructor is a cyclist (member of a local race club), and about half of us in that session ride. His workouts are geared to folks who ride, and do spinning in the off-season. This week, I couldn't make my usual Wednesday night session and went to a Tuesday night class. Different instructor...still a great cardio workout, but definitely more of an aerobics approach than my usual class. I'd still go to that other session rather than miss a workout completely, but given the choice I prefer the cyclist-orientated workout.