Down, by far is the most thermally efficient insulator available. No man made material can approach the range of comfort, ability to compact, longevity,and the overall weight savings of down.
As a paddling guide in the BWCA for many years, and as an avid hiker/backpacker, I have never gotten my bags wet enough to not be serviceable. The answer to the wetness question is a good dry bag, and a solid tent or bivy.
My down bags are older than almost any of my gear. Down will not degrade like a petroleum based, synthetic material (which as a side note is why I prefer Merino Wool, such as Icebreaker to any synthetic base or mid layer or socks). When stored properly it will outlast anything, especially if you use a product like Nikwax, which was mentioned before.
REI's private label bags are great option, my wife has a -5 degree 800ish fill bag that she uses for all but July and August. And we have a pretty big range of temperatures where I live. I have and old Mountainsmith 880 fill 0 degree bag (no longer made) that handles the same seasons for me, along with a TNF Superlight that is older than I care to admit.
If for some reason I had to shop for a bag right now I would start with Mountain Hardwear and Marmot, as they use some of the best down for a reasonably priced mainstream consumer brand.
You dump a lot of $, into a bag, and I must say, its some of the best coin you can spend.
ps
I was in outdoor retail for a long time, as a manager/trainer/buyer, and feel very passionately about people getting the right gear.
Rant over:ar15: