I was using a 3GS until last month, when I finally got custody of my elder daughter's 4! Battery was still strong despite the phone being in daily use for four years.
Comparing the two devices the major upgrades are the increased ppi and iOS7. The 3GS's display looks very coarse now, and iOS6 has a quaint, olden days quality to it.
But the 3GS is still a lovely compact phone no bending here and is absolutely fine for my needs, ie telephony, iMessage/Whatsapp, Tapatalk, Facebook, and a little light browsing.