I remember reviewing one of the early "stretch" bags, a MontBell, eight or nine years ago. Most readers know the concept—the bags are cut a little more generously than a regular mummy bag, then they incorporate elastic strips or some other stitched-in constricting mechanism. The result: a bag that feels roomier but doesn't have a ton of empty space, extra air being difficult to heat up and keep warm. One reason people get cold in sleeping bags is that as they move around they literally pump warm air out and cool air in.
Today such bags include the Sierra Designs 15-degree Sandman ($170; www.sierradesigns.com), a synthetic-fill bag that incorporates the NightCap add-on you mention (a hood that allows you to move your head around without smushing your face into fabric). Another is the MontBell U.L. Super Stretch Down Hugger #3 ($255; www.montbell.com), a 32-degree down bag. Interestingly, Sierra Designs now licenses its technology from MontBell—this after MontBell sued Mountain Hardwear in the late 1990s over a kind of stretch technology used by Mountain Hardwear at the time.
In terms of whether these bags "work," to me the answer is an equivocal: For the most part, yes. They do create a roomier bag without adding too much unwanted extra interior space to heat up. But of course, there's no free lunch. Because they have more material, these bags typically weigh a little more than a regular mummy of the same temp rating. The stretchy MontBell bag, for instance, weighs two ounces more than the non-stretch version (U.L. Alpine Down Hugger #3; $230), to which you're free to say "big deal" without any argument from me. And, it's also more costly to make, so the price is higher—only $25 more in the case of the MontBell, and here again you are free to say "so what."
But, there's no question to me that a stretch bag will be less durable, because the elastic is bound to go slacker than a set of old gym shorts at some point in the future. In the case of a synthetic bag, with a life expectancy of four or five years, that may not matter. A down bag's fill will, however, easily outlive the elastic bands. So you should put that in your pipe and smoke it.
That's a long-winded preamble to some sort of conclusive advice. I've used the Sierra Designs NightCap, and while it's a clever innovation, I don't think it changes bag comfort all that much. I'd go with the MontBell, which is a well-designed bag that's also extremely light. Or, look at a slightly more generous bag, such as the Western Mountaineering's Caribou Microfiber ($230; www.westernmountaineering.com), a 35-degree bag that has a wide, comfortable cut, yet weighs a mere one pounds four ounces.
Check out Outside's 2004 Buyer's Guide for more of the years best sleeping bags.