I used a quilt last night, I have used them on and off for 10 -15 years, and slept in below 0 with a quilt, or with some sort of quilt combo. I also have used a lot of different sleeping bags and mummy bags.
I think a draft collar is worthwhile for a quilt or mummy bag , keeps the temp more consistent through out , other wise you shoulder can be cold, feet hot etc
I dont have any hate for a well done mummy bag , I am fed up with poorly executed bags that dont have smooth zippers, or dont breath well , or are too small. A well done bag (Western Mountaineering) , is what I use more often than anything and many times opened up like a quilt.
Personally, I think with quilts, you can get below 0 with them , but most people are happiest to go to a nice bag somewhere around freezing.