I've only bought Valero gas twice, but both times I got poor performance and terrible mileage.
I had the same experience with their stuff under the previous name of Diamond Shamrock. However it seems to be a car, or model of car, dependent thing. Many people I know swear by the stuff, especially the diesel users.
Interestingly, my previous car, a '90 Mazda 626 did not like Phillips gas bought in San Antonio, but the stuff bought up north in Sherman worked fine, I used it for years.