I was thoroughly humbled my first (and subsequent) attempts at spin class. I'd averaged 300 miles/month on my road bike in the previous 3 years and took a look at the chumps in their sneakers and cotton T-shirts and thought I was going to show them a thing or two. Yeah, just not the same things I wanted to.
The instructor I had was very young and immature and made some very disrespectful comments about anybody who wears colorful lycra/spandex to ride a real bike or who is over the very ancient age of 30 (I am 55 and prefer HiVis when riding in traffic).
The guys in my bike club who take spin classes have shown tremendous improvement in their ability to climb. Me, I am a very poor climber and standing on the pedals I only last a minute or so. The flywheel on the fitness club spin bikes threatens to tear my legs apart at the knees when I forget; the saddle is torture to sit on; and basically the bike can't be adjusted for a good fit.
As much as I hate it, I'm sure that means it's good for me.