I love my apex 5oz underquilt. Can go into the low 40s comfortably before I need to start supplementing with layers. Really easy to work with. It's a little bit more bulky than the down I've seen, but I have had no problem with a large top and bottom quilt inside a ULA OHM with gear and 3 days of food. (still room in the pack for more). Cost was a big issue for me, but the synthetic properties have served me well a few times. I've only used mine for 1 year but haven't noticed any breakdown yet.
http://www.youtube.com/watch?v=EOAb3qGS38s
This is the guide I used for mine
Bookmarks