I started spinning a couple months ago and I love it! In fact, at the time, I posted a thread in here that I believe was titled "Ow, my bum!" and got some great advice.
Firstly, yes, your butt and other areas do hurt the first, second and even maybe the third time. But, a gel seat cover (got mine at Walmart for $20) helps tremendously and each time you go the soreness is less, until you one day you realize that it didn't hurt at all!
It's not the most exciting workout, but I always sweat and feel like I have actually had an awesome cardio work out, which is more than I can say for some other gym activities I've tried. Currently I go twice a week and sometimes a third time on the weekend. One of my classes I enjoy more than the other, but that is because the instructor in the other gets on my nerves (she likes to yell in this screechy voice that just annoys me, but it's the only other spin class that works into my schedule on a regular basis, so I deal).
I don't think I could bike to and from spinning class, though - especially the first few times!
If you end up adding it to your workout routine, you may want to invest in a pair of cycling shoes. I got a pair on clearance at a bike shop and they have helped my feet not to hurt, since they are hard-soled and keep your feet in place better than stirrups.
Oh, one thing to watch out for is your posture. I was gripping the handle incorrectly and developed a minor case of carpal tunnel for awhile.
