the only thing thats PROVEN is that its better for your engine, especially under extreme stress and heat and all that. people say you get better mpg and maybe .5-1hp improvement, but whatever. i run it, just in case. mobil 1
Synthetic Pros: Longer lasting, better cold and better hot lubrication. Under extreme conditions, such as dead cold startup and high performance/high temp conditions, clearly superior to conventional motor oils. Supplied by Porsche in their new cars, that shoul;d tell you something about it's capabilities. Cons: More expensive, although 18.88 at WalMart for 5 qts of Mobil 1 isn't too bad, especially if you follow factory recommended oil change intervals while using the synthetic. For unboosted and/or unmodified cars driven easily and properly maintained w/ regular motor oil, probably not worth the extra money.
I have used Mobil 1 exclusively since it came on the market and in my 2000 ZX3. As hard as I run the car, oil is something I don't have to worry about.