Rating: 5 / 5
I second everything Glenn said above.
I purchased this two years ago, before I really started getting into truly lightweight gear(Salathe Bivy=34 oz.) I'm eyeing a Tarptent but until then this is my "good enough" primary mountain shelter. I use the bivy as a stand alone shelter, without a tarp, on 5 day treks or shorter.
Had a big storm front come in at 11000', Titcomb Basin(Wind Rivers), in September, above treeline. Poured sheets of rain for 10 hours the first night, broke during the day, then poured rain and changed to snow(6 inches total) the second night. At first, I had some trouble with the velcro closures at the top of the bag and had some leakage, but otherwise was just slightly damp on the inside from a small amount of condensation. It kept me dry and warm during both downpours and snowfall.
A third night of cold, moist weather did lead to an increasingly damp sleeping bag, proving the bivy sack is not ideal as a stand-alone shelter for multiple days out.
On warm nights with mosquitos and no wind, the waist-length mesh's ventilation has been a godsend. The flexible, wire-stiffened hood keeps the bivy away from my face.
Certainly not the lightest bivy out there, it is one of the most protean and performs as advertised.