What everyone said!
I haven't done a lot of spinning, but I intend to do a bunch this winter (in the gym in a class). Just the little I did this spring (thanks Veronica!!) DID help both my leg strength and my higher end HR fitness. Really helped for out of the saddle climbing, too.
And, fun!
I also have a trainer at home for the road bike, but I need to invest in some videos. I have a hard time making myself just ride and ride the trainer without outside motivation. Although, just spinning on the trainer for a 1/2 hour in the morning is easy to get in. I'm planning on that this winter to up my a.m. metabolism.
"The best rides are the ones where you bite off much more than you can chew, and live through it." ~ Doug Bradbury