The iPhone 3G was my first device. The difference between that and my crappy flip-phone was enormous. I basically ignored the 3GS because my 3G was fast enough. My anticipation for new iPhone models peaked with the release of the iPhone 4. The retina display and increased speed over the 3G was quite nice. But it wasn't the same as going from cheap phone to 3G.
I'm currently using a 4S. Honestly I can't tell the difference between this device and my 4 except for the home button that actually works every time it's pressed. I've never used siri for anything other than novelty questions. And the count of those is about ten in six months.