I've used this a bit now that things are warming up in Montana.
It's good with a fleece hoody and standard base layers down to around 25. I had it into the mid teens with a DAS parka and was right on the edge of comfy.
The concept however has been great. The bit of nylon seals out drafts really well. Crucial is an elastic loop tied through the loops that came factory with the little cinch cord. I use the elastic to go around my ridgerest, and that means that even when I turn around inside the quilt stays put, and no drafts or cold spots!
I like the narrow dimensions, the simple, no zip design, and I really like the Pertex shell. Resists wind and sheds water like a champ. I've always been a down bag person, but for shoulder seasons in the mountains, this thing is the ticket.