My fitness coach uses them. This is his take on it. "Yeah man, few different theories on it. It's supposed to make your body work (or apparently work) with a reduced level of oxygen available. This meaning that if you can keep the intensity as high as possible with about 50% less oxygen, your conditioning is much much better with normal access to air. Therefore it helps your body process oxygen into the bloodstream as efficiently and effectively as possible. It's had people claim that because you're gasping for breaths when wearing it and labouring to keep filling your lungs up, it strengthens your lungs also... I've found it to do a couple of those things but I haven't measured test results. Personally, I've liked it for the mental challenge that comes with keeping your breathing under control as much as possible. If you start to panic or lose your breathing rhythm, it's very hard not to rip the thing off... On the negative side, I've heard some people say that they reckon it trains your body to only work at about 75% of its optimal output because you're not utilising every bit of your lung capacity. They reckon that because your purposely handicapping yourself, you're repetitively practicing working at 75% of your max... I like it, but it needs to be used on and off doing the same routines so you have the best of both scenarios..."