I have had different experiences and was wondering how much is purely local, and how much might be due to the chains themselves.
Autozone - they are always eager to pull a trouble code for me for free, but they don't seem to know much about actually working on cars, also thier selection is limited. They are everywhere around here.
O'Reilly - They have a better selection than Autozone, the guys tend to be knowlegable, and they accept my used oil. They are also everywhere.
Napa - They seem to have everything, and know what they are talking about, but seem to be more oreinted to the commercial customers that the retail customer off the street like me. There are only a few in my area.
Pep Boys - I've only been in once or twice to get a case of oil on sale. If I ever need an air freshener or accessory doo-dad, they would probably have it. Only one in these parts.