When I'd go on vacation with my parents (cruising was the ONLY way my parents traveled) they'd limit the time one could use the spin bikes to during the prescribed spin classes. So I'd do these classes. Most of the time it was Eastern-European instructors who I'm sure were very knowledgeable about body building / strength training, but not endurance training. Some of the silly exercises and sets they'd have the class do were worthless from an edurnace fitness stand point. Stuff like shoving your butt really far back the bringing it far forward etc. One minute sets at ultra low cadence (Talking, like barely moving the pedals). Sets like this are fine for developed athletes, but (IME) will lead to injuries for the type of people who'd usually do these types of classes. I'd usually bring my bike computer and hrm to record the data and estimate some sort of strain.
Now, when I was in college on the Triathlon team, some of the older students would do spin classes in the gym on campus. We'd do proper interval sets. Classic stuff, 5x5, 3x3x3 etc. These were good workouts. Big issue here is getting a proper bike fit on a very nonadjustable bike. No power meter, but I did manage to get my hrm to connect up to the display. This was nice, but it was better to connect it my computer and just record data there. Another big plus was the fans. If I ever become wealthy, I'm investing in a Big-ass fan. Like the brand name, big-ass. They're awesome and could keep me cool at 175BPM in a big room full of so many other people.