I started working at home about 10 years ago and I quickly realized I had to get out of the house during the day, or I'd go nuts. It took a little longer to realize that when you work at home you are way less active - you just don't walk around as much as you do when you work in an office building. So, I joined a local health club and evolved a pattern of going every work day for a 45 minute workout: 20 minutes on the weight machines and 25 minutes on the exercycle. That worked out great, but riding a stationary bike is boooring. I tried wearing headphones, reading trade magazines, watching the TV in the gym - still boooring.
So, one evening I tried a spin class, which is basically a room full of stripped down exercycles facing one direction with an instructor on an exercycle facing the other direction yelling at you over very, very loud music. I felt kinda silly at first - a lot of the instructors are the "you go, girl" kind of aerobic instructors - but I quickly found it to be a really good workout for 45 minutes to an hour. I'd still rather bike outside, but when the logistics for a real ride aren't there, spinning is about the next best thing - and the fact that every workout is a bit different, plus the loud music and the sweaty women all around you, take care of the boooring aspect.
Here's a heart rate log from a 0530 hour long class I did recently. This instructor (Jan) aims at getting your heart rate up high and keeping it there - she even gets off her bike and walks around making sure everyone has enough tension on the exercycles to be actually working hard, not just faking it. You can see it took about 15 minutes to get my heart rate up above 137, which for my age equates to something called Zone 4. The low end of that is pretty much the fun zone and the high end is the "OK, when can I get off this thing" zone. For about 30 minutes (minutes 25-55) I was in that area and at the end I was just one big ball o' sweat.
All that spinning over the winter translated into noticeable aerobic benefit when I started doing real cycling this spring, but not that much in the leg strength end of things, as far as any improvement in climbing hills. Now, if they would put pedal-powered generators on those spin bikes, maybe I could claim some carbon credits - why don't health clubs have all those cycling/treadmill/stepper machines generating electricity??