Origina iPhone 4 and I for one don't see how people say this upgrade is unimpressive. Literally every aspect of the iPhone 4 is improved or upgraded.
Better antenna (yes, I still find myself holding my phone wrong)
Better camera (front and back)
Helluva lot faster processor and graphics
Larger screen
Native Siri capabilities
Better mic/call quality
All iOS features are available
Granted, there are hardware things that they are leaving out (NFC), but aside from NFC, I can't name a single thing they're doing wrong. Unless you absolutely want NFC and/or absolutely hate the new design, I don't see why you wouldn't upgrade.
iPhone 5S or whatever it's called is almost certainly going to look the same with minor internal improvements (maybe NFC), just a new software feature. At least that's what I'm expecting.
EDIT: I will say this about my iPhone 4.. no real sluggishness (it's jailbroken, but I've restored for a couple weeks to go back vanilla and no sluggishness then either), and it holds a charge as well as it ever has. I don't take precaution and let it die once a week or anything either. But as far as anything being wrong it, there's very little. Mostly scratches on the antenna and back of the phone. Otherwise, it's every bit as good as it was when I bought it, IMO.