I'm a "spin-a-holic"!!!!!
Yes, it can be a bit rough on those fanny muscles until you get used to riding a bike again - but just like when you were a kid and got on your bike for the first time after a long winter, you get used to it and it doesn't bother you anymore. Also, you can get gel seats (a padded seat "cushion") or padded biking shorts if you think it might helpful (many people use them....)
You really need to take 2 or 3 classes to become familiar with the bike ......to learn what resistance levels you are comfortable with, how to pace yourself for a 45-60 minute class...and to learn just what "moves" the instructor uses (ie...sprints, hill climbs, flat-road, "jumps", etc.....) If you are doing the moves properly, you really shouldn't have any trouble with your knees. If something doesn't feel right to you, don't do it! It's your ride - do what is right for you! But also, take the time to talk to your instructor before your class starts and express your concerns. If there is anything that he/she will be doing that may be uncomfortable to you, they should be letting you know in advance and offering you modifications.
I use a heart rate monitor while cycling to monitor myself - my goal being to stay within my "fat burning zone" as much as possible...(actually, I always use the monitor, no matter what type of exercise I'm doing...makes for good comparisons....)....I know I work hard - I tend to be a bit on the competitive side

....but I usually burn anywhere from 450-550 calories in a 1 hour class - which is almost as much as I burn when I run for an hour (about 6.5 miles in one hour....)... there are so many factors that come into play when determining calorie burn...gender, age, weight, heart rate...but for me spinning is definitely one of my favorite ways to work out!