I've got it, and like it. Here's a few things about it.
Get the original altitude training mask, not the 2.0 version. The original has the rubber straps that you can use to give a tighter fit to your head. I started wanting a harder level of resistance than the maximum (red) screw-in and so I did two things:
a. started walking with it on, as making your diaphram work more is needed or even the high (red) setting won't be as much as you need
b. I used super-glue to close a few of the holes that are used to help you breathe out, bc I noticed that the action of breathing in gets tired faster
This has been the best thing that I've ever done to help my endurance, bar none. I've also noticed that I have more energy during the day, and want to move around a bit more. Also, bc of that, my metabolism has increased by a bit and my bodyfat is noticeably lower even though i'm only walking a half mile with it and doing my normal weight routine.
I can't even finish the half mile with it on yet, I feel like i'm about to pass out, that my diaphram is unbearably tired, but I am increasing the distance I can do. Be careful lol.