BoxRec

I haven't personally, but have a few friends who swear by it.

Ive always fancied trying these but would only do it if i was fortunate enough to have a home gym. lol

great for building up your cardio fitness.
ive used mine while doing rowing workouts and some crossfit workouts as well(was very awkward for barbell movements going past your face all the time)
made a massive difference to breathing very quickly.

I've read mixed reviews on this. I've read that it helps greatly. But I have also read that it is a bit of a placebo effect and that nothing really helps in this aspect unless you are actually living where you are training at high altitudes. It is only then that you become truly acclimated to the low oxygen levels.

Whenever I do any cardio work, roadwork/hillwork/cycling/treadmill I always wear my training mask, I set it to 18,000 feet.