good in my book
In the winter I have to take some care to not get overly sweated up. If I have a flat 8miles from home and I'm badly sweated it's going to be really hard to change a flat and continue in spite of having extra clothing with me. I ride if it's over -20F or so and really can't push myself hard on a regular basis outside when it's that cold so a spinning class allows me to really push hard in a controlled manner and when I'm done I'm usually soaked and exhausted. I can work that hard on summer rides but they are typically 50mi and I do the hard HIT stuff toward the end of the ride if I'm going to do it at all.
I loathe spinning on one level but doing it in a class is much better for me than trying to push myself alone at home. Our spinning classes do a good job of pushing me and I try to add to that by pushing myself a bit harder than required. indoor riding is not good for me in terms of building hours on the seat, but it is for doing HIT work and other basic training stuff.