I have done spinning classes for quite a few years in the winter when it gets dark early. The classes depend on the instructor--some are horrible and some are great. A fair number of them have never ridden outdoors and do all kind of things that are useless for training (hovering, and pushups on the bike come to mind), but others do a good intervals session that can motivate you to work very hard.

I like them because I can work hard, they're at a set time so I can't procrastinate, and I've found a couple of good instructors. I avoid or ignore the idiots, or skip certain parts of their workout.

Mark W