Another brand to consider that I've used heavily is Eastern Mountain Sports. EMS has several bags called the "Mountain Light" series. They have everything from 40 degree to -40 degree (F) bags. They are made with 750 or 800-fill down, with a Pertex waterproof outer layer. I know EMS makes some synthetic bags also, but, I happen to prefer the packability of the down. I've slept comfortably in one of the 20 degree bags with a Reactor Liner bag at about -5. I had a fleece union suite on, with another fleece pullover and a watch cap, and heavy wool socks.

http://www.ems.com/

I've camped/hiked with folks using other gear that people mentioned, and they seemed comfortable in Mountain Hardware, very popular. North Face is a "name", but, maybe not the first choice. It seems like it's living on reputation more than performance these days. Even their tents aren't built as well, see the lack of reinforcement around the pole attachments, and some of the plastic fittings.

Make sure you invest in a good 4 season pad. I use a Thermarest ProLite 4. Sometimes I've just used a closed cell foam roll, they can't develop a leak, and actually are more versatile than the self inflating pads, but, I don't find them as comfortable.