I instruct spin class 3x a week. The majority of the participants are guys who have done it for quite a while and they like to be pushed very hard--though there is one woman who usually kicks all of our butts. When I have a newbie I do always let them know to go at their own pace regardless of what the others are doing. We usually tell the new people that it takes about 2 weeks to get adjusted to spinning.
It is well worth it and I highly recommend it. If it doesn't kill you, it will make you stronger.
I prefer to wear moisture wicking material (under armour) and it is completely wet when I am through--so be prepared to sweat. Also, padded shorts are nice if you plan to continue.
Spinning is great cardio workout and as said above will tone your legs. I wear a HR monitor and my HR usually peaks at about 180 bpm during full on sprints. You can also add more tension if you are wanting to build some leg muscle. There are some various examples of spinnng classes on you tube. Check them out to see what it's like and good luck to you.