The sleeping bag I carried was a very old, very bulky Kelty 20 degree synthetic. I'm glad I took it, even though it was bulky, because it was great to crawl into at the end of the cold, rainy days last spring.
I sent it to Maine just before I entered Shenandoah National Park in Virginia, and used just my cotton bag liner and a couple thin poly blankets. They worked great most of the summer, but the second night I was in Shenandoah, the temperature plummeted to the mid 20's! Luckily, there was a housecat hanging out at the shelter that decided to sleep on my feet that night, so at least part of me stayed warm.
I got the bag back in Killington, Vermont...I was glad to have it in the White Mountains!