I was at Costco recently to look at cellphones/service for a friend who was attracted to their "replacement" warranty. (despite the fact that, in general, I despise that place)
First of all, Costco rents the kiosks to a 3rd party who actually operates them.
Secondly, they have (as is usually the case at Costco) an extremely limited collection of devices and accessories.
Thirdly, the salespeople are on commission and pushy.
Lastly, other than the Costco loss/damage warranty, the prices on phones aren't that great, in many cases higher than the carrier's company stores.
I also doubt that if what you want is "great service", you will find it there, other than the replacement warranty, which I hear has (unsurprisingly) a healthy list of qualifications and limitations.
Oh, and there is no way to call anyone there on the phone, you have to make a trip down there if you want to talk to anyone.
No thanks. Personally I think I'd rather patronize Radio Shack than Costco for a cellphone.