[note: I wrote this a couple of days ago, but am just getting around to posting it.]
I've been thinking about the iPhone 5S since it was released yesterday. I'm contemplating buying one, as my iPhone 4 is getting a bit long in the tooth.
But a lot of what's improved won't do much for me. Finally getting Siri would be nice. The extra CPU power (20-40x as much, judging by the graphs) never hurts, but I wonder if it'll really matter. Maybe some games would run without getting as hot, since the processor won't be taxed as much? If so, that'd give a bit of battery life improvement, which is always welcome.
But in terms of meaningful improvements for me? The camera is really the only thing. I like the larger sensor (I shoot full-frame, so I'm of the opinion that it'll only be too large when they can't get a lens in the housing anymore) and larger max aperture. Those will both help in low light. There's some potential for the larger aperture to allow for creative depth-of-field effects, but my suspicion is that the sensor is still too small for that to be noticeable.
I also really like the 720p slomo output. 120p is freakin' awesome. I can certainly think of some things for which I'd love to have that (although a longer lens would be very useful for many of those).
The burst mode and in-focus picking both have the potential to be big difference-makers as well, although it will require some real testing to figure that out.
The dual-color flash will help in low light, too, and probably in a more dramatic way, most of the time. This is really what I wanted to talk about, though. It'd be great to have something like that as an add-on flash for my Nikon. Of course, the coordination between the two would be difficult to mediate. Probably every company would choose its own solution, so flashes become as interoperable as lenses (which is to say, almost not at all). Plus, there's definitely some issue with the detection; auto-white balance on my SLR is pretty good, but sometimes fails pretty hard (this is why I shoot in RAW. I can losslessly correct in post). In fact, that parenthetical leads to the only major change I'd like to see in the iPhone; RAW output.
I don't know what to make of the M7 motion coprocessor. It's certainly got potential. I'd been contemplating a fitbit, so this might completely obviate the need for one. It might also help me to get up from my desk more often during the workday, which would have significant health benefits.
The TouchID sensor... I'm having a hard time getting excited about it, after finding out that the NSA is grabbing data directly off smartphones. So a fingerprint sensor just says to me, "How about I just give you a fingerprint to put into the records". Hard to get excited about that.
[One good thing I read today is that the part mentioned in the keynote, that it read sub-epidermal layers, means that it's much harder to fake. Gelatin copy won't work, nor will a dead finger. Kudos, there. And maybe that's why they bought the company that makes the sensor, for that differentiator.]
Anyway, not sure yet, but I'm certainly leaning towards upgrading. If I can keep my current data plan, that would make it a slam dunk. If not, then it gets a bit more complicated. We'll see.