I use Synthetics only, and I was loyal to Mobil 1 when it was really stand out compare to other Synthetics on the market, and without some crazy sales and rebates that's going on these years.
After Mobil 1 has gone to Group III base (cough cough... I know, don't want to start a war here), and other major brand name is catching up with decent spec (ex. PP meeting GM4718M and HTO-06, Quaker State Horsepower meeting GM4718M, Valvoline Synpower *officially * meeting GM4718M). I am now not loyal to any brand but buying Synthetics based on sales price with or without rebates.
My reason behind this change is because these 5W30 has been quiet close in terms of performance, why not just buy the oil based on sales. Unless I own an European car which require A3 or whatever the manufacture spec (ex. BMW LL-01, Benz Spec, VW Spec...), or I own a very high performance car which needs some robust oil, I am not sticking to one brand as of now.