I would say for aerobic purposes, yes, for skills purposes, no... my $0.02

yes! I think that often a spin class is a better workout than a bike workout, because you don't have any distractions.

If used occassionally for aerobic training, they're better than not riding anything at all. They just can't fully replace outdoor cycling. In the classes I've taken/observed, the riding style is so silly and unrealistic. Yeah, right, we'll all hold a cadence of 120 while we are "sprinting" (weeee!). Also, those monstrous 10-second climbs are real killers. But then mystriously when you get to the "top" and start going "downhill" (which is never mentioned. I guess the "road" is one long climb with false flats) you sit back down and hold that slower climbing cadence as you . . . descend? So we went from almost bouncing off the seat at 120 to mashing at 40. Right. Anyway, if you use them, try to ride the way you would outside. Hold a realistic cadence. Actually put some resistance on the wheel. And when you climb, tweak that nob, stand up and stay there.

[quote=fawcettenator]yes! I think that often a spin class is a better workout than a bike workout, because you don't have any distractions. [/quote]

No distractions? I'm too busy staring at the women in the room to pay any attention to the spin instructor...

For specificity, your standard spin class will fall woefully short and not apply well to time trialing. All the out of the saddle sprints, 120+ cadences, and jumps between 120 rpm and 60 rpm just isn't ideal for training a steady pace. If you're a road cyclist, they have more of a benefit, but still not that great. You'd be better off doing steady state intervals on the trainer (2x20/30 @ 95% FTP, 5x10 @ FTP, 10x5 @ 105% FTP, etc...).