Really it is what makes people like Ford over GM or vice versa, and this verse that, all pure preference. That being said I have had far less issues with my iPhones on AT&T then I did Blackberry, or Android phones on other carriers.
I had Windows mobile devices too, but nothing is as smooth as the iOS and since the MFG of the HW and SW build this, it is meant to work seamlessly together, not so with some of the other MFG's.
I know people who prefer Android over iOS, and Crackberry's over anything based on their usage, but lately I have seen masses around me switching to the iPhone on all carriers, even ppl buying full price unlocked to use on T-Mobile, so everyone has a different answer for why.