For most folks who AREN'T already on a structured training plan, spin classes are great. They get you doing hard interval-like work, and you can go as hard as you want, even if the instructor is lame. WHen they say "GO" you GO! I've done a few spin classes when I was out of town at a YMCA where the average age was probably senior citizen level, but that still didn't stop me from going all out in the corner and generating a ton of sweat.

If you're on a training program with predetermined structured interval work, obviously that will be better than a spin class where you don't know what type of intervals to expect, but otherwise, any time spent on spin class is time that'll help you outdoors on the bike. Watts are watts - you crank the watts on the spin bike, you'll be cranking the watts outdoors.

I tend to avoid any strange bike motions, like the notorious 'jumps' they do (like 1 sec out on / 1 sec in saddle repeating) and any arm motions. Just focus on pushing hard when it's time to GO, and you'll be all good with spin classes. YMCA spin classes are a staple of mine when I'm out of town for more than a few days - much, much better than riding the gym stationary bikes for me.

The motivation factor is huge. I don't know too many folks who aren't die-hard serious racers who can actually adhere to a progressive interval program on indoor trainer on their own, whereas a lot of people are ok with going to a spin class.