I've been climbing PI for 4 months. Haven't tried Fly so I can't compare. It's thicker than Blaze, I have no problems grabbing it for DRT. It seems to wear pretty nicely, I'm climbing frequently and it has barely raised any fuzz.
The sheath stretches, a lot! I milked off 30 plus inches from a 120 ft. length. After that it's settled down. In retrospect I was probably too aggressive in milking it, I wouldn't mind if the sheath had a slight bit more slack or softness in it.
A friend who climbs Safety Blue tried it out and remarked, "it's fast!". To be fair, it was anchored on a 2-ring false crotch so it was going to be somewhat fast anyway.
Haven't tried it with ascenders.
As far as the stealth factor goes, I'm beginning to think that if you need to be that stealthy, you're probably distracted from the task of climbing. It is a very nice color though
-moss