I've never ridden a trainer, but I started spinning when I was in pretty good shape at teh end of a season. My first spinning class kicked my butt- had trouble walking the next day. Yes, they are challenging to cycing enthusiasts. If I were racing, or training seriously, I don't think spinning should/could replace trainer work, but since it's the only option I've got during the winter, I do it (and love it). It's helped immensly with my climbing and endurance. Some of the moves won't do much for you on a bike, but it's a great cardio workout.
And yes, there's lotsogirls.