Excepting how you look like, the mask is limiting the air you get and it is said that increases your vo2 capacity. I know that the luc leger measures your Vo2 capacity, the mask is useless at all? Using the mask increases the training's intensity because you can t get how much air you want and expands pulmonary's alveolas. Some other opinions based on other things than the aspect?
I think that focusing on breathing is the key when it comes to increasing endurance and mental toughening and limiting the air you get is a valuable thing because you will adapt your body to high intensity-low air flow and without mask you can imagine
Celestinmask is just marketing trick to sell your peace of useless rubber. Yes, idea behind it is that limited air flow would mimic high altitude low O2 levels and force your body to make more red blood cells what training in motions really does, but when you train in mountains you stay there for couple weeks and you breathing low O2 air 24h.
There is no any proof that mask could do the same. In fact easily could turn out that mask actually damaging your health.