Yeah, a lot of the guys over at Bike Forums were pooh-poohing spin classes, but I find more credible the ones who seem to be serious cyclists and admitted they got WHUPPED in their early spin classes.

It seems to me that with a HRM you inevitably would get back what you put into it, so I can't see how one would NOT improve significantly with spinning (subject to the time-in-the-saddle caveats that VdeV notes)