I have used kettle bells only twice in my gym, i have to say this the most toughest excersise rotutine i have every done. My back was sore for a week. But i could feel it was getting stronger. The excersie we did was called "snatches" for 3mins for 3 rounds! very exsausting!!!
For info on snatches go here:
Bodybuilding.com - Mike Mahler - High Repetition Snatches: Intense Cardio For Serious Athletes!