I started doing these about 2 weeks ago to replace the interval bike workouts I was doing and I love them. I haven't done this much cardio in years and I feel like I'm doing them for recreation. I've applied the concept to the whole gym and I just wander the gym looking for dynamic excercises to do in clusters, then take the 90 second break and continue. I was also able to get a workout in at home after the gym had already closed using a 75 lb sandbag. Bundled up, went out into the snow and gave it hell. These workouts are why the IPOD was invented.
I'm hooked on these.