Boot Camp?

The Living Social deal today is for 5 boot camp classes for $20 which is ridiculously cheap. I'm thinking of doing it, even though I fear I may die during. Has anyone ever done a class? I would like to be stronger, and obviously be super skinny and all ripped. Should take, what, 2 classes until I look like Giselle? I'm really not sure if my knees could handle it. I imagine it is a lot of squats and lunges which are just tough for someone with wimpy knees. Or if my back could handle it. I'm basically held together with silly string.