I went and did a spin class for kicks. I do alot of cycling, and since the class was full of women I figured it would be a joke. I couldn't have been more wrong; the instructor worked us hard. Its not like stationary cylcing because they try and simulate biking. Lots of riding out of the seat with high resistance, and lots of sprints. If you can keep up with the instructor you will get a great cardio workout, which will help you lose fat. However, like you said, diet is still key.