Spinning is a great way to train in the upper aerobic/anaerobic zones because it can be a good interval workout to help round out your training. That said, I second the opinion that a cyclist or at least someone with some endurance training makes the best instructor. The worst kind are those who treat it like any other cardio choreography class where you spend too much time standing, 'jumping', and overall not training your body to work efficiently on a bike. This irks me too because when would I ever want to stand under load for 5 minutes or more? I wouldn't. Even if I didn't have an ankle injury that makes me really not want to. I have gotten better at spinning faster in a seated position under the same resistance. The good thing about the classes is you can just sit or do your own thing (like not spin at 50rpm when the instructor tells you too--too slow for those knees!).

I recently got into cycling outdoors and I learned a lot of how to train by taking spin classes taught by a racer. Note he was also a good teacher. His 'endurance' classes bored some but I learned a lot (I was a sprinter in a previous life--I could get the intervals thing down). So I started doing longer endurance rides indoors. It's helped me on my new bike outside. So I think that while spinning IS different, it's a fun way to train if you get an instructor you like who plays music you like.