Cardio Coach?
 

I have the first three Cardio Coach audio workouts but have only really used the first one. So far, it seems that the others get progressively more difficult so I haven't tried them yet. Has anyone tried the others? Any thoughts or reviews on workouts 4, 5, and 6?

For those that haven't tried them, I have to say, I love the one I've used. They're great for getting me through a 30-40 minute cardio routine and can honestly be used outdoors, on a treadmill or elliptical or really about any machine of your choice.