I understand it is weird to do such "upgrade", but me and my gf does not really have money for new phone. We don't want to spend more than about 50 USD more. SGS costs about 80 USD in Poland, used. SGS2 is much too expensive. I'll try to find a bit more expensive dual-core phone.
Need help with translation? I can help with Polish language.
Our device is 4 years old. SGS is not a bad phone and I will keep using mine until it dies.
I would still recommend getting something better. Remember that if you buy used phone you might need to buy new battery($10-15) and the headphone jack might be worn. Also some (early?) SGS phones have hardware GPS flaw(Mine has it). It can be fixed at home with some effort(search XDA for the final fix). The AMOLED screen is also a little worn(4 years...). In gray backgrounds you can see how the different subpixel colours have worn at different rate. It doesn't affect anything though just cosmetic stuff.
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?