The iPhone 4S resembles the iPhone 4 almost exactly*, but inside it is a substantially improved phone. As importantly, it introduces Siri, the first voice-control system that I've tried which really does work. When it's working it seems more or less like magic. However, it needs wifi and fast wifi at that to work properly. Overall we are delighted with these phones, and would recommend to any 3GS users out there. iPhone 4 users need to decide how important the faster response, better camera, and, above all, voice control is to them...
The three big features in depth
The iPhone 4S is more than twice as fast as the iPhone 4. This is important, because anything less than twice as fast doesn't really feel much different. It's especially noticeable when taking pictures (virtually no lag), when downloading email, when opening web pages, and when running searches of all kinds. I'm told that this speed hike is down to a 2x speed processor, a much faster graphic chip, and much faster 3G connection. The technical can figure out exactly what difference all that makes. The difference in my hand, though, is that things which seemed sluggish before now feel quick, and things that felt quick now feel instant.
The 8MP camera on the rear of the iPhone 4S is higher resolution than my first digital SLR. However, megapixels aren't everything, and on a very small sensor of the kind they put in phones, more megapixels means more noise, less dynamic range, and poorer colour, all other things being equal. I was a bit sceptical of the new camera. The autofocus is vastly improved, the colours are much better, it's really, really quick, and built in HDR means that it's now capturing shots that really look like they've come from a camera, not from a phone. The price of this, of course, is that the images are now 2.8 MB, which means you'll almost certainly want to scale them down before sending to most people, which slightly wastes the point of having 8 megapixels. If you're not convinced, I'm uploading a picture of some home grown tomatoes -- it's scaled down, but you can still see how true the colours are.
I've flirted with voice control on various things for years. IBM Via Voice, the Mac's own speech recognition, the TomTom, and Dragon Naturally Speaking, Vlingo and many others. None of them have ever really been much use to me. Dragon especially cost me more time correcting what it thought I'd said than I ever saved by using it. With Siri, I was able to send an email word-perfect, make an appointment (and have it offer to call the person I wanted to meet), find stuff on the internet and generally get done what I wanted done, all without any hassle whatsoever.
In a very real sense, Siri 'just works'.
In another sense, it doesn't.
The difficulty is that Siri is using Apple's own computers elsewhere to work out what you've said, so a slow connection over 3G, or over slow wifi, or over quick wifi when there's a lot of traffic on your system (for example, you are running a big TimeMachine backup) means that it goes a lot slower and gives up half the time. Unless you live in a city where there's lots of public wifi that isn't overloaded (I personally find that The Cloud and Openzone are usually slower for most tasks than 3G), don't expect to walk down the street barking out instructions to your iPhone and have it follow them. The noise doesn't seem to bother it very much (in itself quite impressive), but slow connection means essentially no Siri.**
The other thing you can't do is "find me an Italian restaurant", because that feature is currently only available in the USA. You can say "find me an Italian restaurant in central Birmingham on the web" and it will go straight to Google and find it for you, but that isn't quite the same as advertised.
iOS 5 comes with more than 200 new features, almost all of which work on the iPhone 3GS and the iPhone 4 (though not the 3G), except for Siri which is 4S only. I upgraded my 3GS to iOS 5 a couple of days before the iPhone 4S arrived. There are some brilliant new features, especially to do with the camera, a lot of tweaks, and the addition of iCloud online storage. All these work on all three phones. You only get 5GB with iCloud, unless you want to pay more, so I immediately turned off remote backup on all my iOS devices. Still, it's very good for taking a photo on one device and having it on another instantly without having to do anything about it. However, all these are arguments to stick with the phone you've got, rather than go for the iPhone 4S.
A lot of people were very disappointed that the iPhone 4S wasn't called the iPhone 5. I'm not really sure why a different name would be important. Others were disappointed that it had the same case design. For me, a different case design would have been a deal breaker, as it wouldn't have fitted in the built-in iPhone cradle in my car. All in all, this is a powerful, quick device which looks good and feels sleek (possibly too sleek -- like the iPhone 4, it's a bit slippery and probably easy to drop). The call quality is very good indeed, and, thus far, I haven't noticed any signal drop outs - or, at least, no more than you've always had with mobile phones.
Overall, we are very impressed, despite the difficulty of using Siri away from fast wifi.
*the positions of some of the controls have changed a millimetre or so. And you can easily get it in white. Otherwise, that's it.
**update: a couple of days later, Siri seems to be working rather better, including over 3G (but not Edge). Web-chatter suggests that the patchy performance on launch day was due to approx 2 million people all trying to demo Siri at the same time. Still no improvement on the Italian restaurant problem, though.