After using a 4s for the past 2 years, I find the speed of the 5s to be amazing. It's amazing how much of a delay I now notice when I try to use my iPhone 4s again that I didn't see before. After just a couple of hours of use of the 5s, the 4s screen seems so tiny. How did I ever use that?
I now have a new row of empty space for my icons on my home screen. I didn't even think about that and now I have no idea what I'm going to put there.
Touch ID works pretty great, but it's so hard to remember that I don't need to swipe to the left to type my pin code any more. After having done that for 4 years, it's engrained into my muscle memory.
The 120fps camera isn't going to get old anytime soon. It works beautifully.
I see the benefits of the M7 chip right away by trying out the Argus app. It turns my phone into a pedometer (and more) for a barely noticeable battery life cost.
I also know that this was a big deal when the iPhone 5 came out, but I didn't get to experience it since I still had my 4s. The iPhone 5s (and 5) is extremely light. My iPhone 4s feels like a heavy brick now.
This phone is going to be tons of fun to develop on.