I'll jump: I'm looking forward to getting the 5S and the camera is a big part of my anticipation. I take a lot of pictures with my 4S, mostly snaps or documentation shots but some fairly decent stuff I wouldn't be ashamed to show anyone. A better camera that's always in my pocket would be more than welcome.
I remember someone saying: "the best camera is the one you have with you". Thanks to technology, I always have at least one camera with me. While I'd prefer it to be the D800 I can't fit it in my shirt pocket.... And it won't take calls from my customers either.
So this is the first I've even been in this section of the forum...
So I've owned the iPhone 5s for about a month and half now. I have two of the actually (one for work, other for personal), so I do shoot with these quite a bit.
What I have found is in daytime, this is excellent. You may think a lot of phones are great in the daytime, but I haven't found that to be the case. For example my Galaxy S4 seems washed out all the time.
In low light, it's also very good; not as good as real camera, but again loads better than my S4. The flash is interesting, but I have found it to be a hit or miss. Sometimes it will work very well and light my subjects nicely. Other times I find it will measure some of the cor correctly, but then other parts of the picture is terrible. This is bad because I don't post process my iPhone pictures (sometimes I do, but rarely). But for most uses it's great (better than nothing!).
I have found myself on a few trips carrying only my iPhone and not even a camera. Yes the D800e is enormous, but I have do have a Fuji X20 which is suppose to be compact and small. Issue is none of these fit into my pockets without causing discomfort (I only tried with the X20). In fact on these trips I have been able to get the pictures I want, not have to worry about hauling a ginormous camera around (and possible breaking it), and I need to have my cell phone anyways, so that works perfectly.
I'm not saying this phone will replace my existing equipment, just for day-to-day use it is perfect.
I'm impressed with the quality and clarity, but I think I'm most impressed with the AE/AF Lock feature. I didn't see that in the list of features in the original post.
Tap and hold on your focus point for a few seconds to enable this feature, and single tap to disable it. This is especially helpful for focus/recompose, as well as when shooting video where you might pan to a brightly backlit setting.