A down-filled bag works just fine for me here in Oregon. There are many tactics you can apply to keep it dry. You need to have a reliable shelter system that you know how to set up to withstand rain, so the bag won't get wet in your shelter.

When it's raining and it is in your pack, you need to have a waterproof barrier between your bag and the outside world - which can be as simple as a heavy-duty trash-compactor bag used as a pack liner and a silnylon stuff sack for your bag, and placed inside the liner (my current system).

As I just pointed out in a different venue, there is no real difference between the need to keep your down-filled bag dry and your synthetic bag dry, because they both suck at keeping you warm if they are wet.