I've been using a -10C down bag for years now (from MEC- <$150!) and it's never let me down.  I've used it year round- I just use it as a blanket in warmer weather, or use a silk liner as a sheet and forego the bag altogther.  It's packed watertight in a neat compression/rolltop bag, and it's never been wet- totally agree with earlier post re: good planning/packing.

Extremely light, extremely versatile- Best investment in gear I ever made!

(proper storage is key- keep it clean, dry, and loose when not in use, and it should last for decades!)