I do. I don't normally get the latest and greatest, only once, which was the iPod Touch last year in July. I have a 4S which I'm trying to get used to, and trying to put all my stuff there. No space! d: anyway, how about you guys?
I'm on a 4S and I have no qualms with it. I absolutely hated my 3GS, but the 4S was such a significant upgrade that I'm more than pleased. I don't store a crap-ton of stuff on here, so I'm happy with it, but it sounds like you like your phone to hold a bit more data and have more functionality than I do. As with anything, it's all in the eye of the beholder.
I'm using an iPhone 4 because I lost my iPhone 5. It's running great on iOS 7.1.2 and is still an awesome phone. Waiting to see the iPhone 6 but may end up buying a new 5S because I'm not sure I want an iPhone with a bigger screen.
I tend to upgrade almost every release, or every other. My wife on the other hand doesn't want to let go of her 4S, she doesn't want anything bigger. When it starts to get slow and have those kind of issues she might change her mind.