Lyn Greenhillwrote a review of Marmot Atom Sleeping Bag: 40 Degree Down on August 13, 2008
I also purchased this bag for 3-season backpacking and I'm very pleased with the product. I have used it 6 times at different morning temperatures and the 40 degree rating is about right - if you're in a tent. I sleep reasonably warm, and when the temp was below 40 I got cold. Above 40, in a tent, I have been fine. Above 50 you will be unzipping it. I don't sleep in anything except a polypro shirt and underwear, liner socks, and a wool beanie, so if you add a layer of thermal underwear, you can decrease the minimum temp.
As SpriteMV says, the bag compresses very well. I can stuff it in the bottom compartment of my pack and still have room for my tent, footprint, and camp shoes. Try that with any other bag!
The zipper is a bit on the small side, certainly not a #10, so be careful when using it. There is a draw-string hood which works well.
Overall, if you want to shave a couple of pounds from your basic gear set for 3-season camping, the Atom is certainly the ticket.