I too own the zPacks GooseHood and concur with everything Richard said above.
It has small grosgrain loops at the bottom of the front and that back that allow some shock cord to be looped under the arms for a super strong seal. I've used it with a TNF Diez half-zip, Mountain Hardwear Nitro, and Patagonia down sweater vest. I have no complaints about the integration of hood and jacket with any of these collars both with and without shockcord.
It was, in fact, my ownership of exclusively hoodless insulated jackets (except for my shell and windshirt) that inspired my purchase after reading about Andrew Skurka and John Abela's strong preference for hooded jackets.
I haven't slept in it below freezing (my sub freezing sleeping bags all have hoods integrated) but it's kept my head warm during some early mornings around Tahoe that registered in the low teens Fahrenheit.
There's a simple cord loop running from the chin around the face that can be cinched, and I've found it provides a good seal, but I don't know how unique that is to my head shape.
The craftsmanship and finish is excellent.
Blah blah blah just buy it, it's awesome.
