Locally our Meijer has done a better job with the photos than Wal-mart.
It really depends on who is doing the processing and if the machine is well maintained. Some professional places will tweak the settings to give you the best image, others simply take your photos and press the green button (Hope the machine is in order and your photo is already good to go.)
When I've taken family pictures I've always done my own pre-printing processing on the computer to tweak the crop, contrast, color, etc.