I love spinning as well
Unfortunately (...or fortunately ? ) my instructor knows me by name by now and also knows I wear a heart rate monitor so every few songs I'll get;
"Olivia! This is your 3 minutes - GET THAT HEART RATE UP 20 BEATS!"
Yes...I die.
I've been spinning for Oooo....about a year now and it's absolutely fanTASTic for your butt and thighs...and also your abs surprisingly. It's also awesome for improving your lung capacity and heart rate recovery.
I was always a runner but when I started spin as well I noticed a dramatic improvement in my stamina on the road. It's great

Highly recommend
**LittleBigGirl - in spin your leg shouldn't be fully straighened out as there should always be a slight bend or yes, you'll do your knees in...even without a pre-existing condition. If the setting is correct your leg should be slightly bent when your foot is fully at the bottom of the bike. Even when climbing it remains in the slightly bent state.
I used to get really sore knees from it as well until my instructor corrected my settings and then bam - not so much as a twinge!