Well, I received the M50 I ordered from thru-hiker, and have put it to use in a couple of bivys.
I have to say overall, I am very impressed. I was a little apprehensive about the fabric, but I can honestly say I like it better than the M90.
There's very little of the annoying fraying edges that have to be singed with a lighter (compared to M90) . The DWR rolls water right off it coming from a faucet, so I have no doubt it will work well for repelling splashes.
Its not quite as obvious as the M90 for which side is coated. I had to run the pieces under some water to figure out which side to place upwards. With the M90 there is a dull side and a shiny side, and as far as I can tell, the M50 doesn't have that. I have the black color, maybe the others have a more obvious difference?
Breathability is quite good as well, much more then I expected.
It's a little harder to work with then M90, but once I got my machine's tension adjusted correctly, it wasn't too bad at all.