Guru made some really good points.
I would still like to add some of the Kelvin to my cache, but in the meantime, when it gets SUPER cold (like 0 degrees) and I'm not moving, I have to insert an additional insulating layer in there on top of my base layers and under my Celsius. I've still got some old Winona so that is usually what I use. That said, I too have been largely impressed with the Sitka products or else I'd be using something else. I believe the items I bought in '07 I will still be using in '17. Seriously.
One has to remember that this company really started with an emphasis on the mountain or active hunting (elk, sheep, etc.), and I am just amazed at how well their products work for those regimens. Stretchy 'pants' are such a great thing when you're hoofing it great distances or climbing, even if it simply means climbing up a tree or walking through high grass on the way back out of the woods after a hunt. And flexible raingear, and moisture-managing inner layers...
I still think they have a couple of needs to fill for the tree stand guy, but remember that our needs go from sub-zero to sitting in a 100-degree antelope blind. It's a tall order, and like arrowflingers above says, nothing really keeps you completely warm in the worst conditions (at least not while allowing you to still shoot a bow). And Chris's comment above about tight boots is spot-on.
For those wanting to try out Sitka, but not ready to pay full price, now is a GREAT time to find discounts as most stores will want to close out their remaining line so they can restock in '10 with all new items.