I'm a hard goods buyer for a large outdoor store and have been off and on for a few years. There's tons of misconceptions about both down and synthetic.
The pros of down: It's more compressible, lighter, will last longer, and regulates body moisture better than synthetic. That last point gets overlooked too much. Everyone has been in a bag when they started to get too warm. A down bag will allow body moisture (sweat) to pass through it better than synthetic which keeps a down bag from being as "clammy" as a synthetic bag. That simply translates to comfort, but can contribute to warmth as too much body moisture can conduct cold.
The cons of down: It's very expensive to buy good quality down. It does not insulate if it gets wet. This isn't to say wet synthetic will do much better, but the theory is a wet synthetic bag will retain some thermal value although that night is going to really suck. Down does require a bit more love and care if you want your down bag to last. I own 4 different 800 fill power Dry-Loft covered bags that are going on 10 years and I seriously doubt you could tell they were a year old. To get a down bag to last (or any bag) you should aspire to wash it as little as possible. To do that, always sleep in CLEAN long underwear to keep body oil and grime off the bag cover. Synthetic fibers these days are great, but even a super high tech synthetic will begin to lose loft and warmth after 3-5 years with frequent use. Down can not only last twice that long, many nicer bags can be refilled. My Feathered Friends bag from 1994 has been refilled twice.
Any sleeping bag is going to represent the more delicate piece of equipment in your kit. If you treat it as such, you won't get it wet, dirty, torn, etc. I have sea kayaked over a thousand miles with a down bag and not once got it wet. Same for sleeping in countless snow caves while mountaineering. Protect it and you'll be fine.
Regarding down bags, fill power and bag shells. As of this year, I'd say less than 20% of all bags come with a true "laminate" water proof, breathable shell. 100% of them will come with "down proof" shells treated with a DWR water resistant coating, but that won't make them even moderately waterproof. For starters, the stitching isn't taped, so that is a water entry point. Plus, like all DWR coatings that wear off quickly, the nylon will soon pass water with ease. Gore-Tex bags from the early 90's were sweat boxes. The Dry-Loft and eVent bags of the last decade are much better, but an 800 fill Dry-Loft or eVent bag can set you back $400-$600. As an example, a 20 degree Mountain Hardwear 0 degree bag with a true waterproof, breathable shell runs $520 because it uses their in-house laminate shell made of Conduit fabric (like Gore-Tex). The same bag with their non-waterproof shell is almost half that price.
Fill power is the other thing that gets overlooked. In short, the greater the number the better quality the down. 600-650 is the norm compared to 400-500 that you get in most home pillows and comforters. Top tier down gets up to 800-850 down. There's also different types of down from different birdies. Hand harvested Eider duck down is pretty sweet.
Down rules.
