I did some last winter. Brutal, but did me a world of good. I was recovering from a knee injury and was having trouble getting uphill standing on the pedals, my injured knee just wouldn't make it over the top of the stroke. Spin classes helped that because the flywheel gave me a bit of inertia.

Another + for the scenery and this was actually something that made me push harder. My macho pride would not allow me to be seen giving up in a class full of women.

I'm not a racer so not being structured appropriately for training was not an issue for me as it seems to be for others. Still a darn good cardio workout and kept the legs strong.