Seeing anybody using an iPhone 3G or 3GS is quite the rare sight these days. The oldest model commonly seen in public are 4/4S models, usually with varying levels of screen damage.
I sat next to a lady using an iPhone 3GS as her work phone on an airplane. I thought that was a rare appearance (plus the sight of pre-iOS 7 software design.)
My dad uses a 3GS as his phone. I keep asking if he wants a new one, but he keeps telling me that he likes his phone.