Those classes are torture for me. I'll attend if we have a stretch of bad weather keeping me from doing "real" bike rides, but after 10 minutes I'm already clock watching.
The dome-shaped saddle doesn't let the sitbones carry the weight. The handlebars are at an awkward position so my back or shoulders or elbows hurt when I stand.
The flywheel on the bike makes my stiff knee bend faster than is capable of (sensation of knee ripping apart) and I can't get the adjustments just right.
The instructors - some are OK, others are mindless twits that make fun of anybody who is over 30, has ever worn bike clothes or a helmet to ride a real bike, or has fitness aspirations beyond spin class.
I wear a heartrate monitor and find it difficult to average above 130bpm for 45 minutes, max about 150, for spin class.
Last night on a real bike, I averaged 160bpm for over an hour (max 179) -- and had fun.
I'm really dialed into the exact fit of my road bike. The generic spin bike can't be exactly adjusted to match and my body lets me know it.
I know my climbing while standing would improve with more spin classes. And it's better than sitting around. But it's torture just thinking about it.
I bought a fancy trainer and an extra bike to live on it, plus some DVDs. Now, if I can just get the thing set up...