I tried it last year with the thought of using it to help with flexibility. Only made it through about 4 visits before I threw in the towel. I stupidly went after a longer run session and only made it about half way through before I started getting all dizzy and developed tunnel vision. That was the last time for me.
As for the high heat and humidity, I am not sure of the reason other than clearing sweat from your eyes for 90 minutes. I seem to remember something about every move focusing on the heart? Hopefully somebody who knows can explain it because I am interested in knowing as well.