To be perfectly honest: nothing's changed. This is simply the '3GS' to the iPhone 4. It was the natural progression that we all expected; but all that builds up to creating the same impact felt when we first opened the 4S's predecessor. The power is there, the vertically succinct (if ever-so slightly constricting) experience is there, the interplay between iOS5 (read our review) and the structurally sound hardware is there, the image-quality dominant camera is there, the clarification of interaction through removal of choice is there. I don't have to tailor my own usage habits to fit the phone (the main crux of the Galaxy S II), there seems to be a metaphorical blank canvas that paints itself around you. Simply put, to use the utmost cheesy of Apple marketing terminology, it just works.
The design is old; but yet still sets the pinnacle of mobile construction beauty, although mine went straight into case as I got it in fear of breaking the glass back. The software is limiting (and takes notes from Android) but it removes a layer of convolution surrounding getting everything "just right," giving the 4S it's greatest asset: freedom from needless distraction. Siri is a strange aspect to determine: on one hand it has gimmick written all over it, then on the other we had a voice assistant that I actually found myself using a large sum of the time. It's not derivative and wholly unproductive like most of the options in the market; but there's not a whole lot of reason for Siri being there except for finding the funny answers.
Overall, it may not have enticed initially with it's 'minor-ly upgraded' persona; but this has the transferral of technology into pure function and the machined prestige that makes Apple products so very alluring in the first place. Definitely the best iPhone; but not significant enough of an upgrade to warrant iPhone 4 owners to pay. 8
We have a guide on the UK contract pricings.
Also see our test of the Deathgrip on the iPhone 4S.