My friends at Techradar like the iPhone 4S, it seems, and they’ve put together a typically exhaustive review.
Executive summary: if you have an iPhone 4, there’s no real need to upgrade once you’ve installed iOS. If you’ve got an older iPhone, however, the 4GS is a huge upgrade.
I’d like to get my hands on one to play with the Siri voice recognition and see how it copes with my accent, but my car needs an MOT and service. Damn you, reality!