• Compression: takes less space packing
• Loft:  which is basically how it keeps you warm
• Weight: ounce for ounce, degree by degree, you can get lower temperature protection at less weight

The downside is equally compelling:

• Cost:  Good down is more expensive.  There are down/ feather mixes but then you are losing on the above list of pluses.  And feathers tend to puncture the overlying fabric, not a good thing.

• Offensive: Vegetarians find down unbearable
• Cleaning: You cannot throw it in the washer and cannot take it to your average drycleaner
• Dryability: A miserable choice if it happens to get soaked.

Synthetic sleeping bags, are superior also in three ways:

• Cost: Significantly less than even down/feather mixtures
• Cleaning: Throw it in the washer (best to go to laundromat and use one of their big machines)
• Availability: Depending on the temperature range can be purchased from general retailers

The downside mirrors the upside of down:

• Loft:  Inferior period.
• Compression: It is going to take up more space
• Ratio weight vs temperature:  The lower you try to go in temperature, the more weight you will carry

What do I personally use?  Synthetic, although up until I became a vegetarian I always owned down.  After 20 years,  with replacement of the casing, my down bag was still good and usable.  I gave it to a friend.  And this may sound two faced, but I have borrowed it back for winter trips.  Oh well, the geese were already dead.  It is embarrassing to admit, but there it is.  I struggle with imperfection every day. How about you?