Do you mean spin classes?
Personally I find them to be awkward and not conducive to my cycling performance. HOWEVER, they will vary from gym to gym, instructor to instructor etc. What I didn't like was A) Disco lights, B) Happy Hardcore Music, C) Being told to wave my arms around, D) Doing a seemingly random series of "efforts", E) Being told I was not allowed to assume a time trial position (I am a time traillist, I do 90% of my training in the position).
The reality of the situation is that most spin classes are not aimed at cyclists and cycling performance, but weight loss for average gym goers.
Comparing a spin class to a turbo session in your garage is not something that can really be done without knowing how you use the turbo trainer. The way I use my turbo trainer in no way corresponds to my experience of a spin class.