|iPhone 5 running unoptimised Infinity Blade|
First things first, the iPhone 5 felt less of a premium device than the 4S. In the hand it's lighter and feels less tactile. The new back and anodised black antenna (which forms the circumference of the device) don't shout quality in the same way that the glass and metal of the 4S did. I suspect most of the weight and thickness savings have come from a thinner glass front and the aluminium back panel, but personally I'd have taken the old design, a bit more weight and fragility to keep that unique feel in the hand.
Other than the larger screen and new profile (both of which feel a lot less different than mere stats would suggest) the iPhone 5 works in the same way as the 4S; the extra row of icons doesn't change the user experience and the black bars of non-optimised apps aren't likely to be a long term issue; one because app developers will be scrambling to update their apps and second, because unlike the iPhone app on iPad disaster, you stop noticing them very quickly.
I played with Apple's Maps app too. Didn't think that it was too bad either, in 3D mode map tiles loaded reasonably quickly (though I was on a fast Wifi connection, not sure what this will be like on a mobile connection), looked good and everything seemed to be in the right place (although having to say this is a pointer to Apple's current issues).
My overall impression was that whilst an okay upgrade from the 4S, I wouldn't have been tempted to upgrade if I still had one. The market has moved forward considerably and the iPhone 5 seems like a half-hearted upgrade that relies on customers being tied into iTunes and trades on the simplicity of the iOS interface. It marks an interesting point in the symbiotic relationship between Apple and iOS developers.
The iPhone 5 is a really good phone, no-one will be disappointed after buying one, especially if they currently sit in the Apple ecosystem. At the same time I can't help feeling that there are much better phones out there.