I spin during the week days before work. On the weekends I ride. Spinning works very well for me. I don't follow a class routine. I find it has too much intensity variation to compare to road riding. I vary my pace in slower increments and stay at various intensities longer. It works for me. As for bike handling, it doesn't help. It is excellent for on the bike conditioning though. But I suppose that depends on how hard you push yourself. People can fake it in a spin class. It is harder to fake it on the road.
I follow a similar approach, especially during the winter when the days are too short to get in an outdoor ride. A lot of the benefit of a spin class depends on whether or not the instructor is a gym rat who doesn't cycle or someone with real riding experience. A couple of our instructors do nice programs, like the 45 minute climb known as Big Ass Hill, intervals, etc, which transfer very well to the road.
The biggest critique I have about spin classes is that, with rare exceptions, there is no overall plan, nothing consistent or progressive from one class to the next, because they have to accommodate the "drop in" crowd....especially in January
. It's not a structured training program unless you make it so. Good instructors won't care if you don't follow exactly what they're doing, especially if you tell them before the class.
I also recommend a HRM monitor. There's no fooling yourself then.