I have yet to try one of these although I have reached out to one of the companies that manufactures them. The idea behind it, is the body will be deprived of oxygen making the heart work harder while working out an an easier pace. This can save the legs when harder workouts are needed at a higher heart rate. The issue I have with this is two-fold. The first is the body needs easy recovery runs and while your legs will be getting this on days of wearing the mask, your heart will not be. Secondly, any true physiologic adaptation to oxygen deprivation such as at higher altitudes takes many months to occur of living there- not just 30-60 minute exposures as with a mask. Not so sure this is a good idea!
Let’s talk about why masks have been around since 2009 and most people are only hearing about it now, in 2014.
Source: The Truth About Hypoxic Training and Oxygen Reducing Masks