It all REALLY depends on how good the instructor is. Those ones that are basically aerobics instructors on a bike who have you do weird moves that don't resemble actual cycling at all don't help anything except get regular exercise.

I had a great spin instructor who was a MTB racer. His classes were a great workout and helped my riding consistency a lot.

For this year I am taking cycle ops power based classes where there instructors are actually bike racers or triathletes and they are really good at working on cadence and pedal stroke. They're constantly reminding you about form, changing the power setting for intervals, changing the cadence, etc. It's been amazing. I hope I have the $ to do it again next year!