I spin a couple of times a week. Up here it gets cold enough that I don't really want to work at 100% and get dripping wet sweaty. While spinning we do aerobics as well as high intensity interval training. Even doing long hills when outdoor riding I typically try to stay "within myself" and not go into the red. When spinning you can ride as hard as possible until you're dripping and near collapse. I have fat tire bikes (2) and ride outdoors nearly all winter but I find spinning to be a big boon to getting a head start on my summer cycling.