I use and iPhone 4 (conveniently my mic is also broken) and I am not upgrading since I get /so/ much use of it. I feel it can run everything I would want from a smart phone.
My battery lasts like 10 minutes though, and the camera is kind of broken.
I swear every iPhone 4 has their home button and mic broken eventually. I think the battery life dives dramatically after some point which I haven't reached. Will probably upgrade after that happens.
Actually the mic works, I just get/give no sound when in a call. Works perfectly everywhere else. Home button works as well, but I mainly use something like Assistive Touch anyways.
My battery lasts like 10 minutes though, and the camera is kind of broken.