Is this the iPhone you've been looking for?
During very brief hands-on time with the iPhone 5, this much is clear: it's the weight you'll remember more than its thinner profile. The iPhone 4S is already a svelte device: most people probably won't spot the difference if they see the new iPhone from the side.
The screen size, also, is more of a subtle improvement. This isn't a jaw-dropping leap from the iPhone 4S: it's a gradual increase, done almost so cleverly that the front face of the iPhone 5 might, with the screen turned off, look very much like the iPhone 4S. The proof will be in the pudding for how app developers and iOS 6 take full advantage of that extra screen real estate, but the bottom line is this: more screen size and more pixels are good things.
The real killer app on this phone -- no surprise -- might be the iPhone's 4G LTE, as well as the promised battery life. If data speeds and battery life can live up to the promises, those alone will make many want to upgrade.