Spin class is like any other class, it depends on the instructor, and what you put into it. I "taught" spin classes for 2 years myself. They are great in the winter if you're a wuss like me who hates the cold. As many have said though, if you put in the effort, you will be utterly exhausted when you leave this class.

IMHO, nothing replaces riding on the road, but it's better than nothing.