New vehicle, go for synthetic if you want. Good stuff.
Old vehicle, don't do it... I'm warning ya... those gaskets that have been seated with organic oil will begin to react to the synthetic oil and cause leaks. Listen to the voice of TC and Junkpile.
Again, I'm talking motor oil and on a vehicle that has been run with regular oil for a good while. I think synthetic is good stuff, but I would still change it alot sooner than they recommend. It may not break down, but it'll still get dirty. Dirty oil = bad oil. This is why I never buy synthetic, I can't justify the cost since I would change my oil just as regularly as I do with organic oil.