I don't use one because I haven't found any research that supports a ETM doing anything of value. I suppose if you're after the Bane look when you're in the gym it'll help. :laugh:
If you are basically untrained than they will help you pic up a little bit of performance by making your lungs work harder. They will not however increase your red blood cell count which is the whole awesomeness that is altitude training. Not even the really expensive (few grand) sleep tents will help you there. If money wasn't an concern you would have to go for a full pressured sleep chamber of actively live at those altitudes for weeks to get that good good.
Wearing that mask may get you nothing more than scaring small children and becoming the center of a ongoing joke among your friends.
I get why you are interested though.
So......if you've done the research then why are you still leaning towards buying one? Just curious as to if you found something I did not.