So...that's a lengthy preamble to your question about Eastern Mountain Sports' Mountain Light 20 bag (on sale at $159 from the usual $199; www.ems.com). It's a 20-degree bag with high-quality 725-fill down and an extremely light, tough, and water-resistant Pertex nylon shell. That's high-end stuff. And, while it has been a few years since I used an EMS bag, I've always regarded their bags as well-designed and functional—meaning their temperature ratings seem pretty accurate. And, at just two pounds, it's a very light, compressible bag. I wouldn't hesitate to buy it.
Now, the competition—in this case, Feathered Friends' Swallow, a 20-degree down-filled bag that sells for $320 with a Pertex Quantum fabric that's more or less the same as that found on the EMS bag (www.featheredfriends.com). The Swallow is a great bag—I've owned Swallows for 15 years now and think they're maybe the best all-around down bags made. The Swallow is well-made, durable, and has excellent down (about 750-fill). But is it worth more than twice the money of the EMS bag? Probably not. My bet is that in a side-by-side comparison, the Swallow would show itself to be a little warmer. Just a little. And in the long run, probably more durable.
But, all things considered, I'd say buy the Mountain Light—and sleep well.
For the year's best sleeping bags, read the reviews from Outside's 2004 Buyer's Guide.