I used to have a single sleeping bag that I used for summer, winter, Rainier, and lowland camping. It was the REI Mountaineera bag they stopped making many years ago. I dont know what the temperature rating was, as back then bags were basically thick" or thin." My guess is that it was a zero-degree bag. And it was fine in the winter and up high. Everywhere else, it was pretty miserable to use. Especially if mosquitoes were buzzing around and the options were A) Zip up in bag and par-boil, and B) Unzip bag and become a human sacrifice.
So it would be with the Mountain Hardwear Lyell ($350). Its certainly a lot of bag for the money, with 600-fill down, a waterproof shell made with Mountain Hardwears proprietary Conduit material, and a thick collar around the hood to keep drafts out. But its really a winter-only bag. Even an expedition-type bag. Its okay for a winter trip in Rainier, perhaps, but if you climb Rainier or Hood in the summer, then it will be way too much.
So I tend to think an all-around" bag such as what you are looking for is more in the 20-degree range. That will get you around during the spring, summer, and fall; will work well up to 11,000 feet or so on Rainier (during summer months); and could pull light duty in the winter with a bivy bag and an extra set of long underwear.
In Mountain Hardwears lineup, that would include the Phantom ($355), a +15 bag that uses 800-fill down and superlight nylon shell material to drive the weight down to just under two pounds (the Lyell weighs in at three pounds, four ounces). Marmots Helium ($359) is almost identical in every wayweight, temp rating, fill material (slightly higher fill rating for its down), and shell. Or there is Western Mountaineerings Ultralite ($355), a 20-degree bag that shaves the weight to one pound, 13 ounces and features WMs famously high-quality construction. That would be my choice, to be honest.
To boost the warmth a little, add a Rab Vapour Barrier Liner ($40). This waterproof shell goes inside the bag and works by preventing cooling through evaporation off your skin. As a bonus, they also keep your bag drier because skin moisture cant work its way into the insulation and freeze or condense. A Cocoon Thermal Liner ($60) also adds degrees, taking a 15-degree bag down very close to zero.
The 2008 Summer Outside Buyers Guide is now on newsstands. Look for it online soon.