I have a Buffalo Special 6 Jacket & a belay jacket, which I've worn on many Scottish Winter mountaineering trips, in deep snow & strong winds & rain.
It's not waterproof in the 'trusted' sense. However, I've found that when it rains the water beads off the pertex no problem. If you stood in a torrential down pour for hours on end, then yes, I would think the jacket would wet out. The thing with it though, is that due to the way its made, you always feel warm. So, when its wet (my arm went in a river = soggy sleeve) it doesn't feel cold.
I also find the underside of my forearms get quite hot in the jacket, & I would like some sort of vent zip in this area.
The negatives: Whilst its made very strong & it works brilliantly, its a basic garment in terms of cut & fit, 'no frills' as it were. I have the special 6 trousers & I look like a 1940's aviator in them, the thigh is cut too wide compared to the ankle!
The other negative I would say is the hoods, they are an extra item, & again aren't that great, very basic design/cut with a bit of velcro on.
With all that said, I love my Buffalo Special 6, & I've used it & relied on it in some 'very' difficult conditions.
A great combo, is the Special 6, Mountain Equipment G2 Ultimate mountain pants & a Rab Summit or Smmuit Baltura jacket.