So I've been building my home gym and I'm in good shape on weights, I've got a full power rack now and I'm adding slowly. I need a good cardio machine. I am going to give up running because it's killing my knees, this is why I need a machine. I recall a while back, (but can't find it) a discussion about those fan bikes. I seem to recall a bunch of people here saying they were awesome. I've never used one. Can those that have done them educate me a bit? I'm thinking about an elliptical, skier, or one of those fan bike things.