nkfrench 

The spin classes at my gym don't agree with me. The spin bikes have a horrible saddle (can't use your own), there is too much reach to get the saddle and handlebars at the right height. The flywheel kills my stiff bad knee.
The instructor quality varies. One of the incredibly fit and fast women in my bike club teaches some classes and her students do something productive without the nonsense that other instructors do (jumps, hovers). She encourages without being a cheerleader.

One of the guys in my bike club did a lot of spin classes over a winter and found the out-of-saddle work helped his climbing tremendously when he got back on the roads. Others have noted that they had a much easier return to the roads after taking spin class over the winter when they did not to ride in cold and dark.

The advantage of a class is the trainer at home is dangerously close to the couch and TV; or too close to the car keys to get to work early. At the gym there is also the option of using the weights, swim pool, or the other indoor equipment while you're there. The scenery varies more in the class.

I still attend a spin class when weather makes outdoor riding unpleasant or dangerous. I modify the instruction to suit my own needs. I view it as strictly maintenance of existing fitness, not improving on it. I'll sweat a lot but the bikes are too hard on my knees to really get my cardio going.

After one spin class leaving me raw and bleeding from the saddle, I'll need to rest a day or two. Then I'll decide it isn't *that* cold and wet outside and I'll go for a real bikeride.