I don't believe in "wasting" money buying a phone annually. Especially since it would be unsubsidized. Other than the speakerphone, my iPhone 4 still worked fine.
I like to wait until there is some new feature that is good enough. The iPhone 3G was the first 3G capable iPhone, so I went to Apple with that. I wasn't eligible for another subsidy until a month after the 4 came out. The memory limitations in the 3G were a problem by then, and the camera wasn't too good.
Siri wasn't enough of an improvement to buy the 4s out of warranty.
LTE (for when I'm out of town) was enough for the 5, subsidized, as well as someone else needed a smart phone and with only having gone back to work after some time unemployed, didn't want to spend the extra money a contract plan requires.