Like everything else in life, its a trade off.................on the one hand, synthetic bags still maintain some of their insulation when wet, and they dry pretty quick in the sun and tend to be cheaper than down. ON the negative side, they are heavier than down , and will lose their loft with time. (they don't last forever, and also are heavier, and dont pack as small as down)
With down, its warmer, lighter, packs smaller---and if you take care of it, will maintain its loft and last a long time., on the negative side, down is more expensive, and is pretty worthless if it gets wet, is hard to dry if it gets wet, etc.
I used a synthetic bag for years, and a few years ago (after I just about froze to death on a very cold early spring night---long story, dont ask!) My wife bought me a down bag for Christmas---Ill never go back, its so much better than what I had before! Hope that helps!