I've been practicing with interval training for over a year now. I have to say, doing this type of work out, along with yoga, I've seen such an amazing improvement with my health and body.  I feel like I'm 10 years younger again.  This workout at first for me was so insane.  I was instantly hooked because of how difficult it was.  I never really liked going to the gym and I hated cardio.  This workout intrigued me because its cardio and weight training all in one, and the best part, it's only 12-20 minutes.  Sometimes the workouts will be longer if it's rep based.  So I encourage you, if you don't want to pay for a gym membership and is looking for an exercise challenge that actually works, check out this website, http://www.bodyrock.tv.  Here's an example of today's workout.  And yes, it will RAIN SWEAT! It's time for my shower now.  :)