+a bunch to what merlinextraligh said.

I'd rather buy fenders and rain/cold gear for the winter than pay for a gym membership, though.

If I end up having to do a spin class, I'd just do my own thing anyway. Maybe put a Sufferfest video on my phone and go along with that.

One reason they're so popular -- and not even the participants realize this -- is that nobody knows how anyone has set their resistance. Your "10" might be another person's "30" (and maybe a really strong rider's "6"). It's not like a group ride where if someone can't hang or wants to take it easy, they drop off the back. Everybody's still in the room together, spinning away, happily oblivious to whether they're pushing more or less harder than anyone else.