Save your money, look elsewhere and don't be disappointed. The Lafumas are pretty much Euro-trash that don't come close to their stated warmth ratings! You'll see it in product reviews too.
While I wouldn't be so harsh I do have to agree to some extent. In my limited experience based on extensive use of a Lafuma xtreme 600 45 degree bag (and only on a Lafuma 600, which I also got from REI outlet for cheap) I have on occasion frozen my but off when caught out unprotected on a 45 degree night with only a tarp my and a lafuma against a stiff wind. (more below)
I bought one of the X600 bags last Fall on REI Outlet. Rated for +45, but it may do at +60. Would have sent it back, but it will suffice for travel/crashing on the floor and I have used it as an overbag with a +30 bag for winter camping.
Wow, as mentioned i got mine from REI outlet at the same time. I must say though I absolutely love it as a summer bag. Which is what AT trail hikers supposedly love it for as I've heard from talking first hand with the lady at the outfitters in Harpers Ferry. Apparently a lot of north bound hikers buy them as they're just switching over to summer gear in HP.
Furthermore when used in combination with a bivy they do go down to their 45 degrees stated. I use an OR Aurora gortex bivy. Love the combination in fall and spring.
I've even used this combination below freezing, albeit with very heavyweight polartec fleece and a hot nalgene bottle in the bottom. Was very comfortable, but taking heavyweight polartec fleece just to sleep in is actually a pretty silly waste of space. What can I say, I was experimenting.
Here's what's funnier about my parallel experience to @desertdog: I have just purchased a 20 degree GoLite Adrenaline 800+ fill bag, 2lbs 4oz (actually more like 2lb 8oz) and I to intend to use it with the Lafuma 600 / 45 deg bag as an outer / quilt. My hope is that moisture will dissipate right through the down and if it collects and/or freezes it'll do so in the lafuma, the outer layer of the lafuma or my gortex bivy where it'll scarcely affect my warmth. Essentially the lafuma is a sacrificial lamb.
So far I've not yet had cold enough weather to test my Golite. I've slept with it at 35 and swetted. I was pleased, but can't wait to try it well below freezing. Especially with the Lafuma.
==what I love about the lafuma 600 ==
My favorite way to use the lafuma 600 (in the summer) is as a quilt with a foot box. Superb design, superbly light at 1lb 4oz., compact and very efficient. Best of all I can afford to use and abuse it and don't have to worry about it getting damp or wet. For me it's a go to summer bag. After all I do a lot of touring s24o, s48o are my "crimes of convenience" and I fit them in wherever possible. They work well as shakedown rides, are just fun and have taught me to boldly cut back weight. So much so that in summer I travel at 30lbs now. On occasion at 28lbs... and that includes the bike.
s24o summer gear shot w/lafuma (weighs approx 40 w/salsa fargo)
http://www.flickr.com/photos/mmeiser2/4615291196/#very early spring road riding (35lbs on salsa campeon, synthetic 0 deg bag on handlebar):
http://www.flickr.com/photos/mmeiser2/4422555632/#28lbs:
http://www.flickr.com/photos/mmeiser2/4571522634/#**the contents on this last one are pretty much exactly the same as the Fargo. The road bike just weighs a full 10lbs less.
Those bags are a pretty big hit with Appalachian Trail hikers (only weighs 1lb 4oz) and probably do work in +70, 90% humidity conditions, but I wouldn't recommend one for Western US camping at elevation.
I do wonder where you got your information from about them being a favorite of AT hikers. As mentioned my info comes directly from talking to the outfitters in Harpers Ferry. Would be funny if we have the same source, but I suspect your knowledge comes from forums or some online source??
Maybe one of their +30 bags will do you good at +40-45, but for the weight of those bags, I'd suggest something like this - just over $100, though they might not smash down as small as you want:
REI Sahara +30 Regular:
http://www.rei.com/product/798897REI Sahara +30 Long:
http://www.rei.com/product/798898Cost a little more, but so much better than the Lafumas. I use an REI Kilo Flash +40 (1lb 4oz, but not available anymore and it keeps me warm down to the low 30s) for bikepacking and an REI Mojave +15 for cold weather camping. Quite pleased with both bags and got each one for under $100.
I cannot comment on the quality of Lafuma's higher end / colder weather bags based on my experience with the lafuma 600. It might not be the case that they're all so over-rated. Hard to tell from one extremely entry level $35 degree bag. Best not to jump to conclusions.
While I agree with @desertdog completely that the lafuma is a good summer bag for the Appalachians (if I summarized correctly) I have heard that 30 degrees bags are a little to little for western trips in the rockies, even in the summer, which is part of the reason why I grabbed the golite adrenaline 20deg. My source did the tourdivide with a Marmot Atom 30 and a bivy (which I think probably ads at least 5 if not 10 degrees to it's warmth). Perhaps he found this to cold because he started in Banff?
I myself do extensive winter touring here in lower Michigan. I had been using a Slumberjack synthetic 0 degree bag for winter. Way to bulky. Almost took up more space and weighed more then all my other gear combined. Also I think ultimately the GoLite Adrenaline / Lafuma xtreme combination will be far far more versatile and warmer.
Right now I'm lurking here doing some research for a potential winter trip from Michigan to Florida or Austin on the Natchez Trace and roughly following the Blue Ridge Parkway (depending on weather or not it's open)
I love to make a lot of my gear and though I stop short of making myself a mission critical 0 or 20 degree winter quilt I do use a DIY frame bag to supplement my Epic (now Relevate) super twinkie and gas tank, and am currently working on a a-symetrical mid, or pyramid tent design. I guess I caught the DIY/ MYOG bug from the backpackinglight.com MYOG forums. My single favorite forum besides bikepacking.net