I've heard about this and I know a lot of people who train heavily in martial arts and know about altitude training who say that this mask is pointless.
From what I have heard (though I could be totally wrong) in order for altitude training to work you need to really be exposed to high altitude MULTIPLE hours a day...like, probably at least 8 or more hours a day, every single day for weeks.
Unless you are wearing the facemask that much, you probably won't get major effects from it for your cardio, and it's very dangerous to sleep with it on because you could potentially suffocate, even though that might be the only way to have it on long enough to help you.
If you work out with it on you are only making things harder for yourself as you won't be able to run or workout as hard or for as long cause you are getting less oxygen and you could potentially pass out or get hypoxic.
The one effective way I have heard to do altitude training is "sleep high-train low", meaning to sleep at a high altitude or in a hypo-baric chamber and then train at low altitude.
That way, you have the benefits of the high altitude on your red blood cells, but you push your cardio when at sea level so you won't basically be metaphorically working out with a bag of rocks on your back so that you cannot push yourself as hard. That way, the true effects of the altitude training will be felt.
But again, I really don't know jack squat, it's only what I have heard from others who know more than me.