Hi Paul,
I have both the BPL 60 and 180 quilts and find them to be great pieces of gear! I've had the 60 down to about 40 degrees with a Cocoon pullover and was quite warm. (w/o the pullover I started to get a little chilly at about 45 degrees.) I used the 180 on a trip I took to Utah 2 years ago and it performed flawlessly. One night the temps got down to about 28 degrees but I slept pretty comfortable. (I was again wearing the Cocoon pullover and a hat. I was also sleeping in a Tarptent Rainbow.) I would expect the 90 to be somewhere in between as far as warmth goes.
There are only two caveats with the BPL quilts:
1. Narrow Cut – These are cut quite a bit more narrow than the GoLite Ultra or the JRB quilts. I've never had any real problems with drafts, but I sleep in a tent.
2. Cord Thingy – Across the bottom of each quilt is a piece of cord with a cord lock that is designed to allow for "variable girth". I like to put these straps under my pad to keep the quilt in place as my sleeping position changes throughout the night. I found this somewhat difficult to do with the stock cord that came with the BPL quilts. I could get the pad (a Montbell 90 pad) under the cord, but it next morning the loops that the cord attaches to on the quilt showed ALOT of wear. My solution was to get rid of the cord and fashion my own out of some shock cord. I secured some plastic mitten hooks to each end that clip into the fabric tabs on the quilt. This keeps the quilt in place and (due to the stretchy nature of shock cord) appears to be alot easier on the fabric loops on the quilt. It also allows me to use a thicker pad (BA Clearview) if I want.
In regards to using them as overbags…my understanding (from BPL customer service) is that the 60 and 90 were designed to be used as overquilts for the 180 to boost the temp rating. The footbox on the 60 is wide enough to slip over the 180, but when I tried putting it over the footbox of an older Marmot down bag it definitely compressed the insulation. I would assume that the 90 is the same.
Hope that helps and sorry for rambling!