Does anyone use one or has used one in the past? I purchased one a few months ago.. I mainly got it to build up my endurance. I had 7 weeks of training with the Marines coming up and I knew I'd be going on several forced marches with at least 50 pounds of gear on my back. Anyway, it helped me through those weeks and now I use it regularly in my workouts. I max it out with 30 pounds and do about an hour of cardio with it on. Even on my non-cardio days I still use it. Any thoughts?