When I hear spin classes I think of middle aged women in Lycra...
Not always a negative.
I understand that some gyms have sophisticated machines which permit one to measure heart rate and several other performance metrics which might be comparable to Strava data so performance can be measured. Arguably the biggest attraction is the low impact CV benefit.
Spinning and cycling—along with swimming—arguably offer low impact opportunities to mix up a training regime. But perhaps shouldn't be seen as a way to opt out of putting on one's running shoes and getting out on the road.
German track sprint cyclist Robert Förstemann is clearly not a stranger to spinning: