9/10. I just upgraded from an iPhone 4, which I was perfectly happy with, except for AT&T's crappy 3G service in my area. Reception had been acceptable until early this year, when AT&T sold a couple of its cell towers to Verizon (or so I heard). I upgraded in the hope that AT&T's LTE service would be better. It is except, at my house, I only get a 3G signal. I do seem to have fewer dropped calls at home so far, so maybe the iPhone 5's antenna is better, and I do get LTE reception at work.
Other than that, everything is better about the iPhone 5. It's faster. It's lighter. The camera (which was great on the iPhone 4) is even better. I like the extra screen real estate.
I ding it one point for its slick surface. I don't understand why Apple doesn't use a material with some friction for the back and sides, because, "naked," the phone is too easy to drop especially when my hands are dry and chapped from winter. Buying a UAG case solved that problem for me, but why not create a phone that doesn't need an external case? I also wish that Apple would include a landyard/strap connector on the phone, so that I'd feel more comfortable when I'm taking pictures while leaning out over a river or railing.
Otherwise, I don't know what they can improve for the iPhone 5S or the 6. The product matured with the iPhone 4, in my opinion. LTE is an improvement in the network more than the phone (although the phone has to support it, obviously). I'm sure the processor will continue to get marginally faster and run more complex apps, and the camera will get marginally better.