I started spinning clsees last year in late January and have been hooked ever since. Went 5-6 days a week until the road season began in April/May (Ottawa-Canada) and kept it up in the season as well on rainy days or days I wanted a good sweat and did not have a lot of time, or didn't feel like dealing with traffic, etc.
As much as I liked the workouts (and still do) I was disappointed when I started the road season, in that I thought I would be further advanced than I was. My bum didn't hurt as much as other non spinning years, and my stamina was a bit better, but the road beats you up much much more than indoor cycling does.
I was especially surprised at how much better my conditioning was when I got back on the spinner after a few months on the road.
That being said I really like it. A lot depends on the instructor. Some are pretty bad, others are good but their music can suck. And sometimes you get a great class, great music and you really get a buzz.
Also resumed regular swimming lately after a long hiatus. Probably the best thing I have ever done as far as feeling great after. It's boring, but sticking with it pays off. Here's to hoping I stick with it lol.