So they mostly fail by being overheated/shrunk/loss of loft?

You can also boost the performance of a sleeping bag using a set of down trousers and down gilet, which helps gets around the problem of being to warm for the first couple of hours of sleep then being to cold 4 hours later.

I will also bring along an over sized gilet along with me (Big enough to also get my arms inside). You can sometimes get a lot of down for not too much money. My Karrimor Sub zero gilet cost me around $50 in a sale.

http://www.karrimor.com/karrimor-sub-zero-down-gilet-mens-443974?colcode=44397469

Down Trousers are also available

http://www.jack-wolfskin.co.uk/atmosphere-pants-men/1501141.html

They are very useful if you need to leave the tent to go for a middle of the night time pee.

No, the bags were washed and line dried.

Learned about the dryer issue while working with a court program that had inmates do laundry including sleeping bags. They were trying to get things done quickly. When we got the brand new sub zero size long bags back, they looked like summer bags for short people. And kind of lumpy. Like the WW2 chicken feather bags with the chickens still attached. (tip of the hat to Patrick McManus, a fine and pleasant misery

hehe
so, on the line dried formerly sub-zero now summer-only bags,
any guesses as to why they don't work as well anymore? Holes? Just flattening (loft loss)?

You are giving examples of sleeping bag abuse. When you spend significant money ($500 and up)it is a safe bet you will give it much better care. My best bag lasted me nineteen years and was still a sub zero bag when it was stolen. Its successor,bought in 1983, is still giving good service. I still use a double bag (down)that is more than fifty years old.

To be clear, inexpensive 20 degree synthetic bags, used constantly for about 10 months, compressed tightly daily and washed in commercial machine then line dried every 3 weeks, lost enough loft that students became cold when used in shoulder season mountaineering.

The shrinking sub zero bags were another time and place. The inmates also managed to shrink polyester long johns. Interestingly, wool surplus army pants survived the enthusiastic cleaning regime without shrinkage. Perhaps they were already as small as they were to get.

My wife has the down bag, REI, that was her mom's engagement present from the early 60's and it is still the warmest bag we own.

For those looking for inexpensive down and synthetic jackets, I see Uniqlo has some for about $60-$70.

http://www.uniqlo.com/us/

I have probably 500 nights on my oldest Wiggy's bag, and I have it unzipped and use it on my bed as a comforter, too. It's been washed and dried many times and hasn't lost any loft. The company has learned that some detergents can zap the loft but they will replace or repair any bag, no matter how old, forever. I suppose I'm a fanboy but I can't see how you beat a forever warranty in a bag as good as a Wiggy's bag.

That sounds like great way of having terrible balance, ripped and wet sleeping bags and pads. Don't strap thing on the outside. It's terrible for balance and they are not well protected from the elements.

General sleeping bag tips:

Buy a sleeping bag rated for the temperatures you most often expect, not the coldest you expect.

Your choice of ground pad is a major contributor to both your warmth and your comfort while sleeping, and is no less important than your choice of bag. Budget accordingly.

In colder weather, polypro long johns, a beanie, warm socks and maybe a jacket can dramatically lower the useful temperature of your sleeping bag.

Wearing clean clothing to sleep in also helps keep your bag cleaner, and reduces the need for washing, potentially prolonging your bag's lifespan. A Coolmax t-shirt, undies and liner socks weigh only a few ounces.

Down usually lasts longer than synthetics, often much longer if well cared for. When waterlogged, synthetics are warmer than down, although very far from warm. Plan how you intend to keep your bag dry.

Never try to lift up or wring out a bag soaked from either rain or washing; the additional water weight, unsupported, can destroy the internal structures of the bag. Lay it out flat or over something that will support it, like a tent, to dry it.

Thoroughly air out a sleeping bag after every trip, and store it in a roomy breathable bag, NEVER in a stuff sack. Try to air it out every morning during your hike, and remove it as soon as possible from its stuff sack when you arrive in camp each night, which allows it time to reach its full loft and warmth.

Pay careful attention to manufacturer or expert instructions for washing and drying your bag. A mistake here can ruin your bag. Do it yourself to get it done right, and never use a dry cleaner or dryer.

I've never found rectangular bags to be any more comfortable than mummy-shaped bags. Both designs tend to have about the same chest and arm diameter, which is the key dimension for roominess, along with length, at least for me. Mummy bags are lighter for any given warmth rating.

Understand that it is the air trapped by the insulation, not the insulation itself, that provides warmth. Conversely, any dead or leaking air space inside the bag will be warmed by taking away your body heat. That's why snugger is always warmer, at least until it becomes restricting.

A good sleeping bag is an expensive but worthwhile investment. Choose carefully, and be suspicious of design gimmicks and claims of new wonder-fibers for insulation.

In my elder years I started using a large insulating pad and then opened up my sleeping bag - using it as more of a quilt than a bag - with my feet stuck into the mummy foot area. I found that much more comfortable.

I'd read that ultra-light backpackers have started using quilts instead of sleeping bags. Has anyone here used one?

I think I'd want some kind of a foot pocket just to allow me to kind of hold the quilt in place and thus to minimize the size of the quilt.