I spent nearly two weeks finding the perfect bag (or best bang for buck), and was steered away from Big Agnes. I ended up with a Marmot Helium, which is hugely suggested by many of the Appy trail / ultralight backpackers. 850 fill goose down with huge, lofty baffles, 1lb 13oz (under 2 pounds), compresses into a TINY bag (but comes in a nice huge cloth sack to alleviate loft loss) and rated to 15 (tried and true from the many reviews). Often I find that most mummy bags leave little room inside, and I'm a bit claustrophobic; the Helium has lots of room inside, surprisingly (I'm told BA bags are meant to do that as well). I've spent the last four nights out in it, and while the temps have been in the 30s, I've had to keep it unzipped. I have a huge problem with circulation when camping. Even when I was a young scout (and had what I thought was extremely good camp gear), I could never get my feet warm at night, no matter what I did, no matter what the temps. With this bag I've had no problems with that, even with my head uncovered completely. This is the first sleeping bag I've felt adamant about, and I've had countless.

Sleeping bags and tents are two places skimping doesn't get you very far. If you intend to do a fair bit of touring, bite the bullet and spend nicely - You won't regret it.

Edit: No matter what bag you go with, a pad is a necessity if you wish to stay warm.