Interesting that all the different opinons are correct
.... The summation by Chris is quite complete. Down - for comfort, synthetics - for ultimate safety. I love down - it is comfortable, soft, breathes better, more compact. I can't explain this, but despite waterproof fabric, down still feels more breathable. As to the compactness and weight - with the current technology synthetic fibers, like the latest Polarguard fillers, weigh the same and are almost the same compact as down. I can pack my +1C (35F) 2005 Polarguard synthetic into the same very small bag, size of a small loaf of bread, that came with my +1C down bag. The word is, - synthetic bags should not be compressed that tight, - this eventually results in irreversible shrinkage, so I pack it in its stock bag, size of a big loaf of bread, which then is somewhat compressed anyway when loaded into my clothing drybag. Down bags won't deteriorate from repeated excessive
compression (and synthetic bags won't deteriorate for years from normal compression in their stock bags). As to the safety, - synthetic bag needs to be stored in a drybag anyway, same as a down bag, and with a drybag chances that down bag will lose its insulation properties when wet, are pretty slim - mostly this may happen when tent leaks.
I could've used my down bag many more years, but had to buy a synthetic one, because down somehow screwed up. After a few manual washes in bathtube the down became "lumpy", and no matter how I tried to break those lumps through the fabric, nothing helped. As a result, it got "bold" spots in the insulation, and didn't protect me well in cold nights.
So, if you are okay with sleeping under something not as soft, light and breathable as down duvet (even the best sythetic insulation is still stiffer and more airtight than down), - synthetic fibers are fine. You can get some pretty good last-year or discontinued +1C synthetic brand names with the latest fillers under $70 (and those for warmer temps - under $50).
Combination of synthetic and down - I donno... they are not cheap, and as I can see, not as popular as pure down or pure synthetic.