I have an original iPhone, and I've already posted elsewhere about how I'm looking to get away from AT&T because of coverage issues where I live now, so my views are from a purely academic perspective I suppose because I wouldn't be upgrading to the new iPhone either way (unless they'd announced a new carrier partner :lol).
My main issue with the 3GS is that it has some nice new features, but none of them really fundamentally fix some basic problems with the way the phone's OS operates.
A new camera with video is all well and good for those that would use it, but they're using voice dialing as a way to force people to buy new hardware? That really rubs me the wrong way - that functionality that's been in phones for years is being used as a way to persuade one to do a hardware upgrade purchase. IMO, voice dialing is a matter of auto safety and usability and SHOULD NOT be a matter of sales.
Some of the 3.0 features are nice, but my main gripe is still with the antiquated pop-up system used for notifications. Say what one will about the Pre, but it's implementation of such things is far better and more robust than Apple's on the iPhone, and one would think Apple would see the utility in the way its designed and, quite frankly, steal it.
I surmise, however, that it's use of pop-up notifications is relegated to the philosophy and view of the OS. It's not a multitasking OS (for the most part), so you have a "home screen" that you return to rather than a central "desktop" area. On the Pre the notifications can come up separately and not demand attention because they're another item on the "desktop" of "cards". The pop-up notifications on the iPhone though, since there is no multitasking desktop view of the OS, are items that have to be dealt with immediately upon arrival and you either have to exit the current activity and enter the proper app to deal with it or just ignore it. It really makes doing anything on the phone when you have people texting you completely frustrating. What's more frustrating is that it's not clear whether Apple even realizes this is a frustrating implementation, because they're babbling on about how wonderful it is that now developers can willy-nilly throw notifications at you from their apps! Great, just what the doctor ordered.
Anyway, added hardware features are nice, but I really find the iPhone OS feeling a bit clunky and a bit long in the tooth at this point, and I wish Apple would spend some resources addressing some of the underlying usability issues.