I find that I can actually get a more intense work out on my own on a spin bike than I can in a class. I think it all depends on how much one gets into the group dynamic. I find it a bit distracting.
+1. Even in the winter, I will take the cold over indoor spinning but when the weather is too ugly, I will train on my trainer, rollers or a spin bike, but not in a class. I like to control my warm up time, interval and ride lengths .