+1 on Marmot
+1 on Mountain Hardwear

As always, do some research. There are a lot of good websites with reviews, discussions, etc. out there. The type of use you want the bag for is very important:
-Dry climate vs. wet?
-Compressable or bulk not that big of a deal?
-Light weight needed (personal carry) or weight OK (vehicle carry)
-wide temp ranges
-Use with second person (zip together?)
-Hasty camps (tactical) vs. deliberate camps (backpacking, camping, etc.)
-High visability (blaze orange) vs. concealed (camo)
Those are just a few things to consider. I'm sure you can think of other issues/considerations that are particular to your situation.

And, generally do go with a bag with a rating higher than you think you need. You'll a) get a margin of safety and b) compensate for the manufacturer's overly optimistic claims.

HJ

I don't do a lot of camping but wish to keep a sleeping bag for bugout purposes. Due to my location, I am forced to consider owning more than one bag to deal with temperatures anywhere from 0-100 degrees F. I'd like to have one bag that will accommodate liners that easily zip in and out instead of multiple bags. Does such a bag exist? If so, would this be the most practical approach?

Just as important as the sleeping bag is a quality sleeping pad. Don't kid yourself. The ground can suck the warmth out very quickly.

Therm-A-Rest brend is top quality.

Cameorn.

You can find bags that "layer." See http://wiggys.com/category.cfm?category=6 (and search for FTRSS) for example.

You can also "adjust" your sleeping bag by DIY means:
-You can buy a bivvy sack that can be used as an outer shell for colder nights.
-You can wear long johns or double long johns for exta warmth. Don't forget socks, gloves, a neck gaiter, and of course a hat that covers the ears.
-You can purchase various types of liners. Silk is light but expensive.
-On summer nights, with a full zip sleeping bag, you can totally unzip the bag and use it like a blanket.

Those are just a few ideas; I'm sure others can point out more.

Snugpak have a bag that will do that called the Special Forces combo bag. Its actually two very capable synthetic bags which zip into each other using a centre zip baffle

http://www.snugpak.com/index.php?MenuID=93-101&ItemID=74

http://www.polimil.co.uk/acatalog/Snugpak_Special_Forces_Combo_System__166_18.html

Probably the limiting factor for most people in choosing a sleeping bag is going to be price, that would be the first question to ask your friend. I'm sure everyone wants a top quality -20 degree, 800+ fill, 1.5 lb bag, but nobody is willing to shell out $600 for it. There are a lot of really good bags in the $200 range, but it might be overkill if you only want a car camping bag.



Are you really going to be using a sleeping bag in 100 degree weather? Even a summer bag can be pretty warm in 70 degree weather. Instead of getting multiple bags, I would just concentrate on the lower temperature ranges, and if it's only for emergencies use only, I'd really only be concered about anything below 50 degrees. Now you've just cut your temperature range in half. In addition to what Jim said, there are a lot of things you can do to adjust the comfort level. The shelter and the sleeping pad are equally important, and can add or subtract to the temperature rating of the bag. A full length zipper allows me to use my 15 degree down bag in 60 degree weather without overheating, or some extra clothes, a good hood and a liner can bring it down closer to 0 degrees. So one bag can cover a pretty wide range of situations. Some people even boil a hot bottle of water to keep next to them while sleeping, or use those chemical heating pads.

I ask those who have more knowledge and experience so that I can make wise, informed decisions. Thanks for the excellent feedback.

Wiggies makes some fine bags, one of which I have been using for nearly 20-years.

Once again my degree in anthropology and multi cultural family puts ETS on the cutting edge of worldwide web diplomacy. Is your friend orthodox, secular, somewhere in between or just wants to sleep warm? Would He be breaking a mitzvah climbing into a sleeping unit that is shatnez?

You don't have to go to a Tijuana upholsterer to find strange things in your gear's lining!

Depending on the situation there is wide range to chose from. Even as a hot sleeper I would not recommend any bad rated over 32 degrees. If its hot, your hot no matter what your in or not in. If its chilly or cold, i like 32 at the min and then working down.

FEATHERED FRIENDS, in Seattle is by far the best sleeping bag I have EVER seen. They routinely outfit peak and antarctic expeditions.

Mountain Hardware is also very good.

I personally use Big Agnes. They are middle of the road as far as price and manufacturing. They cut weight on their bags by taking loft out of the bottom off the bag and including an insert for a sleeping pad. This way the bag is lighter and it never slips out off the pad. The pad is in it.