People I know chose quilts for their JMT hike ...and Enlightened Revelation custom 20 for him and a katabatic (850 fill duck down overstuffed to 10 for her) Quilts save weight and sometimes money for the same warmth.  They have a more flexible temperature range but can be drafty if you don't use them correctly.   They are probably not great below about 20 so winter use is dubious but for 3 season use they can work very well. There are lots of custom options making it hard to decide if you have not experience with them.  Even "cheap" they are a $200+ purchase so you want to feel like you are buying something that is right and is going to work for you.

You use them with an insulating pad (eg Therma rest x-lite R-value 4.2 is a good 3 season pad).   The idea is that the down you lay on has no loft so is a waste.  A sleeping bag's long zip is not necessary since the quilt's back is open and the bag's built in hood only works if you are a back sleeper and is only necessary if it is very cold so the quilt eliminates it. 

There a number of styles.  Sewn food box, zipped foot box, full zip  etc. Also different ideas about the paneling and down distribution and pad attachments (helps with drafts).  Instead of a hood you can wear a beanie or a down "helmet" in cold weather.   Some even wear down booties and gloves.

I will probably get a quilt when I upgrade my current bag for something lighter... a value Kelty lightyear 20 650 fill down bag (~3.5lb)...and expect it to weigh about half that bag for a 20 degree quilt slightly depending on the fillpower and sizing I choose.