I first encountered the green whistle back in 06 when i had an ambo ride and had wanted to try it ever since. After years of thinking it was pethadine i finally came across a mate that had a few vials a couple of years ago and here's my verdict -
Personally i think it's a lot like ether (which makes sense seeing how it's a halogenated ether) but better. You still get that N02 like effect and the "brain disconnect from body" drunk sort of feeling. But unlike ether, which to me felt like my body was almost forcefully ripped from my mind, it felt like my mind floated away from my body. If i had to compare the 2 it'd be like a nice 40 year old bottle of wine compared to goon, that's what penthrox is to ether. Tastes better, feels cleaner and more enjoyable (rather than dirty) and doesn't kill you're sense of taste and smell for the next few days...
Tho i did see some weird shit while on acid - i would get stuck in these temporary reality loops that would only progress frame per second till a full cycle was complete and then would continue like normal as if the loop never occurred... indecently that was the last time i had acid, wont go back there again :S
But other than that, i love the stuff, wish there was more around