Some people may differ but my philosophy is that I would rather be hungry than cold. Meaning that I would rather shave weight on food than a sleeping bag. Not sleeping well is exhausting and demoralizing. I don't ascribe to the old infantry "travel light, freeze at night" any more.
Also remember to calculate the weight of whole sleep system. For instance if you go with a light weight bag that isn't as warm you may want to bring along some heavier sleep wear, whereas with a more robust bag you can sleep in your silkies.
I use a heavier bag with bivy but I partly make up for the added weight by being able to use a tarp instead of a tent.
leo

I am an avid (aging) backpacker, camper, climber in all season and ALL areas desert, mountain, deep South, cold North. I have.......5 or so higher end bags here, from down/Gortex, to nylon/down, and one Mil. sleep system. My 10 year old is using it. I "stole" it as well, like $45 for two of the bivies (one digi, one woodland camo), one summer (green part) and on winter (black part) and two stuff sacks at a local flea market. I like it. Yes, a little heavier than some but I would have spent 3x's as much if needed. I like the versatility of it. I had spent 200 days a year for 4 years as an Experiential Ed. counselor and equipment manager for the camp, think Outward Bound for troubled teens. I know alot about most gear. I never had synthetic bags as the older versions didnt last near as long as down, today they are better.

I would not hestitate to pick up a mil spec sleep system if the opportunity presented itself. I would also encourage you to get a mil poncho liner, sew a long zipper onto two sides. This accompanied by a bivy can be good down to 40 degrees or better (depends on how warm you sleep).

Ironwood

If you are going to use a hammock, there are some diy solutions available, as well as an astounding number of custom jobs like this.

Be interested to know if anyone has experience with hammock applications for camping.

I use tents, rather than hammocks, since I often pitch in treeless environments, but there are plenty of backpackers who prefer hammocks, and there are several manufacturers - Hennessy is the most well known brand, carried by REI (along with Eno), and there are several cottage manufacturers. I believe there is a forum dedicated to hammock camping, but I have never logged on.....

If using a hammock, there is always the Snugpak under blanket, which can be used in conjunction with a centre zip sleeping bag such as as the Snugpak Special Forces 1,2 or Combo.

The higher loft down bags will normally compress down to half the size of a synthetic bag and can weigh half the weight but normally cost 2-4 times the price.

The trick is to get the down bag on sale. You might even find them at unusual retail outlets.

I have picked up a couple of excellent Vango Down Bags (2 and 3 season bags) at half their RRP i.e the price range for a good synthetic of a similar temperature specification.

Keep an eye out for the down bag bargains.

The local Tesco supermarket chain over here in the UK, carries a cheap Down Bag for around $60 but with some caveats. Its a narrow bag suitable really up to a 40 inch chest and would be comfort rated to around 5 Degrees Celsius or 40F so really a 2 season bag. It is small and compact though and almost competes with something like a Vango Venom 300, which normally costs 3 times as much with its 650 fill power (UK) or 700 fill power (US). Weight is similar to the Vango but the Vango will get you another season for the extra $15 it cost me.

Edit - I've also heard that the Tesco down bag has occasionally been on sale for less than $30. But unfortunately will be no good for me as the bag is just to small.

I have one of the miltary sleep systems, MSS. As others have said, very heavy and bulky. The Gore-Tex bivy cover is great, as long as it comes to you in servicable condition. The system is not something you are going to want to pack very far, too heavy.
I mostly use mine only for car camping. I di use the GT bivy on outings that I use a tarp instead of a tent though.

The price is good, but if you look around, I have seen them as low as $120, new in the packaging. That way you are sure of the condition.

One more thing, since this is a military item and there ae those that will buy anything military, that also means there are imitations out there. The genuine system is made by Tennier Industries.

if you are car camping, or sleeping in the backyard in an emergency, then any good sleeping bag from an outdoors store (or even K-Mart) should work. Just check the temp rating. If you expect to sleep at freezing temps, get one rated down to -10 degrees.

the issue of bulk and weight comes up if you are backpacking the gear. at that stage anything bulky is a huge hassle. I use a good goose down bag for those purposes. More expensive, but much better for reducing weight and bulk. Remember to keep your down bag very dry, or you will be an unhappy and cold camper :-)

Pete2

I purchased an MSS mostly because a sleeping bag and shelter was one part of my gear I've been neglecting for too long and I wanted to get something that is well known and reliable but also inexpensive.

I don't go camping or hiking often enough to justify spending so much money on smaller and lighter gear -- it's what I can afford and it works, so I'll deal with the weight for now.

I have mine set up in a Hellcat style Alice setup (Alice pack and frame with Molle straps, kidney pad and MSS carrier).

Don't forget a good groundpad, and keep in mind the extreme cold end of the rating is for 4 hours sleep and also assumes you're wearing polypro base layer + cap. It's not going to be comfortable but you'll be alive...

+1 pad. I'm getting the insulated 4" one.