Spinning I've tried in the past, but I didn't find it really transposed into cycling - especially if the instructor tries to do all that "hovering" and upper body stuff.
Add fascile instructors (some, not all), un-needed, terrible and over-loud musak and the terrible (sometimes dangerous) state of the bikes - "we maintain all the bikes to the highest standard" = bent pedals, broken straps etc, and the fact that you can't generally use clipless, then I didn't feel that it was worthwhile.
Wattbike sessions (no music, knowledgeable BC coach, are another matter entirely. Brilliant! I'll be re-enrolling next year (they only run through the winter)