I've been using Marathon gas, almost exclusively, for about a year. It just happens be a really convenient location. My truck seems to run the same as it did when I used Chevron.
I've had issues with a Marathon and a Speedway station (same company), but I have no reason to suspect the gas itself as the problem. It was more likely bad handling of the fuel during delivery or problems with the stations' tanks. I used to use Marathon/Speedway on a regular basis and never really had a problem besides what I noted above. I use Shell now, but that's only because of the 5% rebate Shell credit card I have.
