Berber,

If you're planning to thru-hike, you may want to consider using two bags. Assuming you are starting in March, the first couple of weeks can see very cold nights, and then the same at the end, but mostly very warm for the middle months.

On one of my thru-hikes, I used a good down bag (probably rated to about 20 degrees F) and then used a fleece bag liner most of the way. I went back to the down bag in New Hampshire. I was using a fairly heavy bivy bag over it. I can say that I was quite comfortable the whole way.

On another trip I used a Western Mountaineering bag that was rated at about 35 degrees F the whole way, but used it more or less as a quilt most of the way and sometimes slept on top of it, but used every stich of clothes in my bag at the beginning and end. Again, I was using a bivy bag, but a much lighter one.

The bivy is very good in shelters. It keeps bugs, mice and wind off the bag without taking up extra space. Also, If you have to camp away from the shelters (it seems to happen to everyone at some point), all you need is your poncho.

Hope that helps. best, jcp