Something else to remember about HIIT is that whatever activity you're using, you should be working so hard during the high intensity phase that you couldn't sustain it for any longer than about 30 seconds max. Also, you should feel like you really "need" the recovery period. For example, if you're running sprints for your intensity phase, after your 30 seconds, you should have to walk to recover. If you can continue to run, you didn't push hard enough during the work phase.
During a workshop I attended for my personal training recertification, one of the presenters gave a great session on HIIT. He said that many people confuse HIIT with straight interval training. HIIT should be so intense that you could only do it about 2 times per week. He said that straight interval training is also very effective; the difference being that on the intervals, you work very hard during the intensity phase, but you could sustain it for a minute or so, and during the recovery you could continue moving at a fairly rapid pace. So, the IMAX workouts are a high level of interval training, but not really HIIT.
Hope this makes sense.