I also have the Kelvin vest. Great as a layering system under my Ascent jacket. Very warm and light weight. I believe the newest version of the Kelvin series is quite a few ounces lighter than the original stuff.
I picked up a Cabela's Teewinot earlier this spring on closeout for around $45. It's 100Gram Primaloft One insulation. I figure it's a few ounces more than a pound and I can compress it to the size of a cabbage ball. I was confident I was going to get a spindrift until I found it. It does not have the intergrated stuff sack pocket like the spindrift.
Also note that the Kelvin uses Primaloft Sport which lofts about 16% less than Primaloft One. By my calculations Primaloft Sport is roughly equivalent to 550 Down and Primaloft one, 625 Down.
If you are an Ounce counter I think the Spindrift is the ticket.
If highest warmth is your criteria then i think the Kelvin is the choice.
If you are on a budget the Teewinot is a good choice.
Patagonia Micro and Nano puffs are also good quality options.
I currently have the spindrift (actually wearing it right now in my office ) its a great lightweight layer but with the kelvin having 170 grams of primaloft compared to the 80 grams in the spindrift i figured it would be a good step up for my next layer when it gets cold.
I used the spindrift ALOT last fall and it was great but just not enough coat for the colder days sitting and glassing for sheep in late Oct and Nov.
I had a chance to look at the Kelvin vest and it looks nice so figured I see what guys who have used it thought about it. Thinking the pants may be a good layer to have as well...thats one part of the system that is weak, an insulated layer for my legs when i am sitting glassing. With the zippered bottoms i could just slip them over my boots and put them on quick when i am glassing.
I have used the Kelvin Jacket for 2+ yrs and pack it religioulsy into late November. It has served me well, though does burn very easy i found out I wear it under Kuiu or something else as it is not a sneak type item with the finish it has. It definately has more loft than the Spindrift and would serve you well in those colder day in October and November.
For glassing it would be perfect. It is very warm. The only time that I have been comfortable hiking in it is when the wheather is about 10 below and the wind howling. But at those times I am glad that I have it.
I wish Kuiu would come out with a more insulated coat. The spindrift is simply not warm enough for my uses, too thin. I want a puffy coat that will keep me warm when stopped to glass in cold temps. I would opt for the Kelvin stuff before the Kuiu spindrift in my case.
I beleive sitka is making thier new kelvin this year with primaloft one. A puffy jacket, down or synthetic is one great piece to have. The kelvin is pretty heavy, but its got a bit more insulation. Id also look at other mountaineering companies. They may not be camo, but for a layer like this your going to want something over it anyways.
Iv got an arcteryx puffy jacket that i really like.
I've used the Kelvin jacket. I find it to be over priced. It isn't very durable. I am usually an XL, but needed a 2XL, and it was still tight in the shoulders, short in the arms, and way too big in the bottom.(I realize that fittings vary for each person, but I usually have not problems. I am 6'1" 200 lbs. average build.) I have found a valuable resource for gear, particularly new gear: Captivatem Outdoors. Mike Monnin is the owner, and the guy you talk to on the phone. He lives in Barrow, Alaska. He has turned me on to a Montbell jacket which is about 1/2 the price of the Kelvin, fits incredibly well, and is as warm or warmer than the Kelvin. I don't know the weights, but the Montbell packs smaller.
The one draw back is that it has a nylon shell, so probably not quite as quiet, but more durable than the Kelvin.
I would highly recommend the Montbell for anything in Alaska, and sheep elsewhere. It probably isn't an ideal outer layer for elk or deer as it is a bit loud, however it is thin enough to use as a heavy layer in extreme cold.
I use some sitka stuff, and I like it. Particularly the Stormfront rain gear, and the Coldfront. Lawnboi stated that Sitka is revising the Kelvin, I would concur that it needs it. If you get it to fit you alright, it is probably an okay jacket, but I think you could find something just as good for much less money.
I own quite a bit of Sitka and the Kelvin pieces are my favorites. After being stuck out overnight on a sheep hunt without much gear, I always have a Kelvin vest in my pack. It packs up so small and is incredibly light and it is phenomenally warm; it will get me thru anything. It is always a layer on my deer hunt sits as well. For high country hunts where I am going to be sitting and glassing, the Kelvin jacket get the nod. They make two version in weight, I can't remember the weight difference but I have the standard Kelvin, not the light. I see for guys that want to use it as an outer layer, they are coming out with a Fanatic Vest this year.