I agree with the others who have posted here who have worn Sitka gear quite a bit. I have a lot of it and think it is great stuff. It has been on my body for moose hunting in the northern Yukon Territory of Canada (close to the arctic circle) to deer and hog hunting in south Texas. The clothing is very versatile if used as it is supposed to be. If you are very active the Celcius, 90%, and Ascent lines of clothing, along with the appropriate base layers (with the Core top used as the only layer in hot temperatures), are perfect for temperatures from 15 degrees to 100 degrees Fahrenheit. When you start sitting down for long periods in cold weather or get into REALLY cold weather when active, you will need a real insulating layer like the Kelvin vest, jacket and pants. For extreme cold, like when hunting polar bear, forget Sitka gear or most other lines of clothing! That requires highly specialized clothes.
I have found the Sitka gear clothing to be very durable, but it will eventually wear if subjected to enough abuse, just like almost any other clothing. It is fairly quiet, but not nearly as quiet as unlined fleece or soft wool. The big advantages of Sitka gear are its light weight, slim fit (which is great for bowhunting), ability to breath and transport moisture (sweat) from your body to the outside, ability to shed water, ability to dry quickly, and ability to continue to provide warmth even when wet. Other materials and systems do some of these as well or better than Sitka gear, but nothing does the combination of all these things to the same extent as Sitka gear, at least that I know of.
Allan