I've been doing it for the last year of so...But I'm out of it at the moment with a damaged knee. Not the best endorsment
Don't know about other clubs but at mine we start off a 2 hour lesson with about 30-40 mins of aerobic stuff. Loads of jogging on the spot interspersed with different sets of push-ups...Gets the heart going.
Depending on the level of the lesson we have some sets on the heavy bag. 3 sets of 3 mins. 1 min light and fast, 1 min hard as you can, then light and fast again. Then a min break and onto the next set.
Then a load of sit-ups and then stretching.
After that the actual lesson begins. We either do sylabus for our next grading or we just do some routines. Depends.
Then some light sparing at the end.
I would say that it is really great fun. Gets you fit very effectively. Its better than the gym because its competitive, you push yourself to do better more than you would pounding a treadmill.
Its all taylored to your ability and the instructors will push you, but not into doing stuff you simply are not capable of.
The only advise I would give is do some aerobic stuff at home in fron of the tv got get warm and do some extra stretching. Make sure you are warm though! And don't over do it! But it really does help you kicks if you work on your flexibility outside of class...
Anyway, hope you enjoy it.
Pete