I've actually never thought of AT&T as 'mediocre' because of a Starbucks relationship. I view them as 'mediocre' due to a decade of terrible, unreliable, and spotty cellular service. It's amazing what service we'll put up with just because all of our friends and family are using that same provider.
And it's rather peculiar that we all associate AT&T with 'mediocre' thanks to their Starbucks wifi sponsorship.