In reply to Basemetal:
I've got one. It's pretty good. Easy to adjust to cut out the wind, or to ventilate, as needed. Hood is insulated, and surprisingly good, for military kit. Hood folds away when not needed, so it's not flapping about the place.
Not warm enough to use on its own on the high fells in winter conditions, but over a warm baselayer, I've not needed anything warmer in the Peak District so far this winter. Maybe a 3 season garment then?
Only issue might be the length of the arms. They're just about long enough for me, but they are short compared with any other garment I own.
Never had a Buffalo Special 6/ Mountain Shirt, so can't really compare, but I've got a Buffalo windshirt for fell running. The pertex used on the Halo is thinner, lighter and probably less windproof than the stuff used on my Buffalo.
Hope that helps.