When using synthetics for really cold weather, it takes a lot of it which means extra weight, and an awful lot of extra volume. (Need a big backpack). Also, synthetics don't last as long as down, they tend to loose a slight amount of loft each time it is stuffed, which eventually shows up in the long run.
However, if you don't backpack (car camp) the weight and volume are not an issues. And, synthetics are cheaper and easier to make.
Where synthetics really shine is in long distance hiking situations, particularly in humid environments (your PNW). Down will slowly absorb moisture from both the air and your prespiration and eventually collapse to uselessness, whereas, synthetics will loose only some of their ability to insulate. If you are a weekend hiker, this isn't a problem, but if you are out for a week in cloudy weather where you cannot dry out your quilt during the day, synthetics are highly recommended for safety.
I have both. My down quilts are for cool and cold weather, but I'm not a thru-hiker. My synthetic quilt is only one thin layer and is for summer nights that are just cool enough to need something. Quilts this thin, in my opinion, just aren't practical to make with down and, they are so small, the weight & volume disadvantages are negligible.