No spin classes. I'd consider a spin class if the bikes included full data -- power, heart rate, etc. -- and was run by an experienced cycling trainer. Unfortunately the only such place near me closed down shop last year. All the other spin classes I've checked into are more oriented toward generalized fitness with way too much sugary cheerleader stuff from trainers.
But I'm a pretty diligent self-starter with exercise. I did begin using a Cycleops trainer last summer after a car hit me, breaking and dislocating my shoulder. Then I was diagnosed with thyroid cancer. Between the two I did very little outdoor cycling for about half of 2018. But using the indoor trainer and following along with some training videos helped me set some patterns for training that I still follow.
But I grew bored with training videos and switched to binge watching TV series. I set my phone app to use a timer to notify me of timing for interval training, mixing seated and standing efforts.
It kept my fitness up remarkably well. Probably within 75-85% of my personal best just before the injury and illness. By the time I resumed cycling outdoors the main challenge was regaining my balance, bike handling, and aero position. I'd been accustomed to sitting up on the indoor trainer, so it took awhile to get accustomed to the drops again. But the spin training helped improve my short duration high cadence in real road cycling, so I feel more confident up to 130 rpm for 15-30 second bursts. Before using the trainer I tended to bounce in the saddle too much at high cadence.
And I'm back in physical therapy for the shoulder, neck and back issues. It's helping.