The flat out sprints are just making you work harder, if you've ever done a proper session it can leave you on the floor gasping for breath before you've even made it half way through.
After using it as a training method at rugby, we were told you'd probably burn the same amount of calories as a reasonably fast jog that took double the time, but it raises the metabolism higher than it would be by jogging after the exercise so you burn more calories in the hours after the session.