The theory behind it is a load of **** really, they reckon that during a 2k your actual breathing muscles experience fatigue which affects ur performance - the only problem with that is that your breathing muscles work non-stop and have been for the whole of your life, they're almost as resistant to fatigue as your heart muscle(which never fatigues) so I don't really see how its going to help.
That said, I have used it, and it does make you feel like you can breath better etc. but more likely to be a placebo affect than anything else.
I wouldn't use it if I were you, if I were an Olympian training for that extra 0.1second in my performance than I'd maybe try it out, other than that its absolutely not worth the money.