I do spinning classes a bunch during the winter. It's certainly an improvement over doing nothing. Every instructor runs their spin class differently; i.e., some run it like it's an aerobics class (which sucks) and some run it as if you're doing real intervals (which is awesome). You have to check out a few instructors to see how much you like their class "style".

Resistance on a spin bike feels very different from "real road" resistance.

All that said, I like spin classes during the cycling off-season at least. It's something different, helps keep things fresh, and they're fun (if you can find the right instructor).