Well I think the intensity thing is a matter of how much one gets caught up in the vibe of a spin class. Now they are essentially fixed gear bikes, so there is no coasting per se, but you can still slacken off and let the free wheel push you along.

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.