Josh: "The problem with quilts in cold weather is your shoulders get cold, but your jacket will offset that."
That's not necessarily the case. It depends a lot on how the quilt is designed, it's just that a lot of them aren't designed for truly cold weather. I have a Katabatic Blackwelder and an MLD Spirit 30, and I've not had that issue with either of them, because they both have drawcords that allow you to snug them up around your neck quite nicely. Being designed for colder weather, the Blackwelder is quite a bit puffier, so in addition to sealing your body in pretty well it also covers part of your neck.
Alec: " Moreover, they seem (as far as I can tell, and someone correct me if I'm wrong) that they aren't meant to be drawn up completely sealed under you. That bottom opening simply doesn't look like it can be closed completely."
You are correct, it doesn't seal up completely under you. It's not intended to, as you theorize. If you draw it up nice and snug, it will cover what your pad doesn't.
I haven't tried Katabatic's attachment system yet. I've just been using the webbing straps. I run them across the bottom of my pad, so the edges of the quilt wrap around the pad, then I snug up the draft tube around my neck. I can roll around without letting drafts in when I do that.
You're also correct that Katabatic's quilts don't open flat. That's really the only thing I wish were different about it; if it did open all the way I'd probably end up using my Blackwelder a lot more, mainly because it ends up being overkill in moderate conditions.
That said, it's been great when I've camped at high elevations and in snow.
The final decision is still going to be very personal, but some of the quilts out there will work impressively well in some pretty harsh conditions, so they're still worth a look.
That said, I'll probably be investing in a -40 bag for Arctic expeditions sooner or later. :)