Would be spinners,
I am a spin instructor and team sponsored cyclist. Taking the cue from my students, the most important component of the spin class is the instructor. As some of us tend to favor certain types of music and workouts vs. other instructors, I would suggest to all that you "shop around" for a good instructor who will give you what you are looking for, i.e Van Halen and interval sets that blow you to pieces...(my favorite).
As for the benefit for cyclists, again, your instructor is the best indicator for this...are they a cyclist? what kind of shape are they in? are they experienced exercise physiologists?
The unfortunate thing is if you belong to a club and paid the dues already, you seldom have the choice to shop around as this may require you join another club..ouch$$. If you are not getting what you are looking for, I have some great advice....become an instructor yourself; that's what got me into it.
As a competitive cyclist I have found that most students prefer to "learn from a pro" rather than an aerobics instructor. Best yet, about 25% of these students now own bikes and do rides with me.
For those looking to spin, it is a great workout and can truly help your performance off season and even during your racing months.
If you are not satisfied with what goes on in some spin classes, saddle up and teach others what you know. It is a rewarding and fun part-time job.