I actually just did a spin class last night. My "No Boundaries" running coach did the class for us and it was so much fun. I haven't done one in a long time, but really liked it. The best tips (that came from my coach last night) would be:
1- Give it a couple classes so you really get the hang of it before deciding whether you like it or not. He also said to try a few different instructors until you find one whose class /personality fits you best and that will keep you motivated to keep going.
2- Don't compare yourself to anyone else in the class as everyone will always be at different levels (good advice anytime, really...) Go at a pace where you are challenging yourself, but not where you won't be able to walk tomorrow.
3 - Don't cheat yourself...don't keep your bike at the easiest setting but then pant and breath hard so everyone else thinks you're really kicking butt (I thought that was hilarious

) or click your gears up and then back down again to make everyone else think you've gone way down in your gears (again...

almost fell off my bike) Why would you go to the class then?
4- And his last tip/statement was that yes, the feeling in your butt will return...at some point. But he also said to make sure that we were standing up once in a while when we needed to (whether the instructor said to or not) to keep your butt from falling asleep.
Hope some or all of that helps. Again...it all came from my coach, not me! But give it a try...it really is a great work out. Don't forget your towel!