iPhone 4 for me was the best release. That's why I'm still using it with no desire to upgrade at all. I have thoroughly used the iPhone 4s and 5, and I am not impressed after these 3 years. There were no major changes that interested me. Local VoiceControl without the need to use Siri is a big deal for me (only the iPhone 4 has it; although, you can put back with jailbreaking). Retina display is still great; have changed the battery for a brand new one two months ago, so I still have all day battery life - and the speed is more than decent for me even though it's jailbroken and FULL of fun stuff.
But the major drawback? I also found the sound quality from the iPhone 4 to actually be BETTER than the 4S and 5 - and it's a pretty common finding on hi-fi forums. It's something I couldn't believe when I tested all 3 out. But I guess the major reason why I'm not upgrading is the fact that I use my iPad 3 and Nexus 7 more than my iPhone. I use the phone for music, calls and messages. And music is about 85% of its use. So, upgrading will be kind of useless.
p.s. Some people in my family are very, VERY, happy; however, to be using my unused upgrades.