I got the Winter INc on order from Stormcrow based on reviews like Hobie's, and impressions of it's versatility like Knotty's. Plus I have a Paul Gibson 2/3 Lost River for marginal temps (50-70F). But I had no issues popping for the Winter Inc as opposed to the 3 season Inc, which is only marginally less expensive for quite a bit less down/loft, imho. but if someone is a gram weenie for thier UL backpack, I suppose it makes sense (although more clothes or fleece may be needed with a 3 season in very cold, negating the weight savings).
Awaiting my WInc, nonurgent, Storm.