I did a quick search on these and didn't find anything other than two gear swap posts, so here goes.
Just a few minutes ago I was looking through STP as they have a 40% off coupon going on, and I stumbled on this:
It's a down sleeping bag by Mountain Hardwear, semi-rectangular cut that can be unzipped all the way to a quilt, weighs 1 lb 13 oz, and costs under $160. The part that really caught my attention, however, is the "switch" feature from which it gets its name. The sleeping bag has different loft heights on the "bottom" and "top" which can be switched (i.e. you just flip the sleeping bag upside down) in order to warm up (35-degree side) or cool down (50-degree side).
Personally, I think this is an excellent idea, especially coming from a such a large brand that generally doesn't cater to ultralighters. Of course it will never be as light or flexible as a simple quilt, but I can see a lot of benefits to having a bag like this if you're not quite ready to make the switch to quilts. It provides the ability to adjust to the temperature simply by flipping the bag over, and you can unzip the bag all the way and use it as a quilt or even share it with your significant other. It also caters to those who dislike the cramped nature of mummy bags.
Have any of you guys ever used something like this? What are you thoughts?
Also, in the middle of writing this I added it to my cart to see what the total was with the discount and it ended up being just barely over $100, so I bought it and am going to try it out.