Y'all make some good points about regular oils and regular changing routines, that's just good practice. Where a true synthetic shines though is in its properties. The lubricity is far more consistent due to uniform size of the molecules, which are also less affected by temperature. You get better flow, less gunking and no build-up. If you switch a high mileage engine over to synthetics you should change the oil and filter at about 1000 miles as it will often cause the sludge and build-up left by regular oils to break free, potentially clogging the filter and/or oil galleries.

There is also the friction reducing properties that is an added benfit of the synthetic structure. After a complete switch to Mobil-1 I usually notice about a 2 - 3 MPG increase in economy in most vehicles. It made a big difference with the V-10 in my '01 F-250.

If you want a clean, well lubricated engine with the benefit of economy and longevity then synthetics are the way to go. If you're driving a flat-head V8 with loose tolerances or some other older vehicle, then don't worry about it. But with modern engineered drivetrains getting the most available power from tighter tolerances and pushing thermal boundaries, then the choice becomes much clearer.

You can watch this thread go on and on, back and forth, but if you want the real answer look in the real world. Mobil-1 has a product line that is very diverse: Racing, Aviation, Agriculture, Industrial etc. Pure synthetic lubricants meeting and exceeding the demands of some of the harshest and extreme environments people and equipment can operate in. Those are facts, happening everyday wherever you care to open your eyes and look.