I've done quite a bit of camping in all types of conditions/situations, and I would strongly recommend getting a synthetic bag unless:

1) You will be camping in sub-freezing temperatures where maximum warmth is needed and the bag is unlikely to get wet (frost build-up on the exterior surface due to condensation can be brushed off in the morning)

2) You want to absolutely minimize weight. This would be in an ultralight approach where you are counting every ounce, and you're willing to sacrifice some degree of comfort for the weight savings.

Stuff gets wet. Condensation is the main culprit, but you can also accidentally spill something on your bag or get it wet in countless other ways. Two days of constant rain and you can bet that all your stuff will be damp and clammy.

A synthetic bag will still provide some warmth when wet, and will be much more comfortable. Hang it up for 20 minutes in the sun, and it will be dry. As has already been stated, down is useless when wet, and it takes a really long time to dry.

Obviously keeping any bag as dry as possible is key to comfortable camping, so have a dedicated dry-bag stuff sack for your bag. Don't plan on storing the bag and tent in the same dry bag. The tent fly will almost always be wet with condensation in the morning, not to mention if it's raining.

Last year, I did an ultralight-ish three week tour, and I brought my three-season (25-30 deg.) down bag simply because my only other sleeping bag at the moment is a heavy four-season synthetic bag. I had a couple days of fairly constant rain, and after that I was ready to have REI fed-ex me a synthetic bag. And I would have done it had the weather forecast not been favorable for the remainder of the tour.

A good synthetic bag is only a few ounces heavier and slightly bulkier than a comparable down bag. For a long tour, the extra weight and bulk are well worth the benefits in my opinion.