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Found out on a recent trip to Crater Lake that my current bag just isn't going to cut it for below 30-ish temps. The last week or so I've been doing some comparison shopping on various internet gear shops, but I wanted to take a look at some close up. I stopped in at our local outdoor store (Peak Sports, Corvallis, Or.) and took a gander at what they carried for bags and pads. It just happened, I stopped in on the same day a rep from Cascade Designs (MSR, Thermarest, etc....) happened to be there. He showed me some of the key features of their sleeping bag line, and I have to say.......I'm kind of leaning that way. Since this is primarily for the Oregon and Washington Cascades, I'll be going synthetic. I know its a trade off for weight, but if I get soaked up on Adams or Pyramid and freeze all night....no bueno.....

On another note, I'd picked up a Big Agnes FlyCreek UL1 before my run to Crater Lake, and I have to say it performed very well in cold, snowy and wet conditions. It takes some getting used to for getting in and out since there isn't really any headroom, but once you're in, plenty of room to keep you off the sides. In the morning, slid over to one side and had plenty of room to fire up the JetBoil and make coffee.

The big difference is not weight, it's size when compressed. Which may not matter as much for thin bags designed for temperatures barely below freezing, but makes all the difference in the world when you need a real winter bag.
In those conditions, you don't need to worry about liquid water at all, of course.
I was once nervous about down in above-freezing conditions, but I've found it's not hard to keep a bag dry if you only use it inside the tent, and the savings in pack size is still enough to be really appealing. Say you want to do some technical or semi-technical climbing, or some bushwhacking: a smaller pack helps.

The only reason to go synthetic is for cost. But a quality down bag is worth the money. If you're concerned about humidity, get one of the DriDown (and other brands) models.
Down is lighter, packs smaller, and lasts longer. But only the high quality stuff. If you're getting 550 or 600 fill down, you might as well get a nice synthetic.
Splurge on an 800 fill down beauty, you'll be happy for years.

Cost is the main factor, and primarily it will be used in the cascades for spring and early summer. There was a considerable weight difference between bags of the same rating, like 2lbs10oz for down and 4lbs for synthetic, but I can deal with that. Main thing was getting feedback from anyone who has used the Thermarest bags, because honestly I like the design and they seem well made. We'll see, probably pull the trigger on one in the next few weeks.