I have just been looking to get my mum into the iPhone market second hand wise and was torn between 3Gs and 4 and 4s, after lots of evaluating i ruled out the 4 as the signal issue was a pain when i had mine and then i looked at 3gs - 4s and the fact the 4s is a really powerful bit of kit with a camera which out performs her current compact won out.
For myself the speed loading webpages/handling apps/ image/video quality mean it really is something i had to have.