I borrowed it to get acclimatized to high altitude for a trek I am contemplating in March.
And I was also curious about how well it works for endurance training, which I won't do until the summer.
Using only the mask (no tent) I spent 3 days (set at 8,000) gradually using it for longer periods. Thought I would start slow.
From a total of about half an hour (3-10 minute sessions) to a little over an hour (2 - 30+ min sessions) with some minor exercise (chair squats) and a little acoustic guitar playing for fun.
Although, during use, I experienced no problems or shortness of breath. A couple of things happened later.
On the third day, a nasty little headache developed later and my average resting heart rate over 24 hours went from low 50's to mid 60's.
(and it did nothing for my guitar playing, which is only at the 3 chord level in any case, imagine Darth Vader playing "I Saw Her Standing There").
The following day it remained higher (above mid 60's, asleep) than normal although I stopped using the system.
I have a Garmin and it monitors 24 hrs. Perfect for my level of OCD.
HR returned to more usual levels after another day and a bike session showed negative side effects.
In all seriousness, who has tried these machines and what side effects did you experience.