I have heard over and over: It does not simulate altitude, but it DOES strengthen your lungs and the surrounding performance. It will not effect your blood oxygen levels like it claims, but it WILL improve your cardio and breathing for combat sports. I have friends who have them, including friends who have since the version 1 came a year or so back, and they all say it has improved their breath control noticeably.
ALSO: I have consistently heard claims that this product improves handling asthma greatly, I have heard first hand and read second hand that the strengthening of muscles involves makes breathing during asthma attacks much easier and manageable. Something to consider as opposed to steroidal inhalants for strengthening(I dont know much about asthma but just sharing the possiblity)...