I don't personally use them because I decided after some research on my own that it probably wouldn't benefit me much. I have a friend who has used/uses one, but I haven't asked for a full review from her. Last I had asked her though, she had been using it for about a month and was "eh" about it. She hadn't noticed any difference in her workouts (running, cycling, rowing, etc.).

One of the main reasons I opted to save my money was because it doesn't actually mimick higher elevation training. It basically just makes you breathe with restriction.It will help you strengthen your diaphragm since it has to learn to breathe against resistance. It does cut down on the amount of air you can get at one time, but the "make-up" of the air you're breathing is exactly the same with or without the mask. (I.e. if the air outside is 50% oxygen, it will still be 50% oxygen with the mask. You'll just been getting 25% less air with each breath. The thing with elevation is the air is actually compromised of less oxygen, not less air overall. I hope that makes sense.)

(I don't know if that's the kind of info you were looking for or not, so I hope that answered your question.)