Some suggestions to spin class instructors from an old grouch:

Don't tell me to smile during an out-of saddle sprint
Don't expect me to whoop and holler
Give cues on how long each sprint, climb etc. will last
Make sure the bikes are well maintained (loose crank arms and sudden loss/increase of resistance are regular problems)

Otherwise, it's a great and enjoyable workout, especially when winter weather keeps you off the bike. As mentioned earlier, a lot depends on the instructor. You can get more variety by going to more than one.