I have been a HUGE fan of synthetics for years. Living in new england, stuff can get soggy sometimes. I currently own two sleeping bags that I use regularly. One is a down 20* bag, which works fine almost all year round. The other is a polarguard 3d bag from EMS, rated to 35*. As compressible as down is, the synthetic packs smaller.
Some disadvantages of synthetics are that it breaks down easier, MAY not be as compressible as down for the same rating. However, they are fast catching up. Disadvantages of down are if they get wet, they are useless (although, sleeping in any wet bag isnt fun). They require a little more care, but generally last longer than synthetics.
With all of the advancements in synthetics, given the choice, I would go with man-made materials. You get what you pay for though; for a good bag, expect to pay a pretty penny. Go through discount outdoor stores for good deals too. campmor, sierra trading post, etc, all offer decent deals on bags. Good luck!