Review Apple’s design language is easily recognisable: clear glass or plastic that’s spray-painted from the inside, a frequent use of aluminium, stainless steel and any colour so long as it’s white or black. But the new iPhone takes this familiarity further, by making the iPhone 4Salmost identical to last year’s iPhone 4.

Apple's iPhone 4S: external tweaks are subtle, the main changes are internal

Of course, it’s not the first time this has happened – the 3G was followed by the matching 3GS. But for the millions who had been eagerly anticipating an iPhone 5 since, ooh, June this year, it came as a big disappointment. So, was that disappointment justified or should you upgrade without hesitation? Read on.

The cosmetic differences are few so you won’t need to hear much about the iPhone 4S’s design. It’s the same slim, solid and classy machine with components precision-tooled to the micron, not the millimetre, and the same buttons as every iPhone: a home button, volume, ringer and power buttons.

Some older cases might not fit due to a slight shift of the buttons

One change is the position of the black lines on the steel band that edges the phone. Those black lines are functional, and are indicative of a whole new antenna. This aims to address the dropped calls and spotty signal of last year’s model. Not everyone suffered from a poor signal but if you did, then you’ll be pleased to hear the new set-up works splendidly, giving the iPhone something which performs like one of the strongest antennas around.

So it reliably makes and receives calls – hurrah! Signal strength and battery life, the bare minimum for a phone, are both strong with clear voice quality. Daily charges are still recommended but it won’t need a lie-down by lunchtime, unlike some smarties. So what else is new?

The camera has been overhauled optically and captures stills at 8Mp

The only other cosmetic alteration is situation of the volume buttons and ringer switch. They’ve also moved just enough to mean that some cases, including early editions of Apple’s own bumper, won’t fit. The buttons match the positions on the Verizon iPhone 4 sold in the US and newer bumpers are designed for both versions. But if you’re planning to recycle last year’s case, depending on when you bought it, it may not fit.

Other changes, and there are several, are all internal. First of all, there’s a new processor. Well, not quite new, it’s the A5 chip found in the iPad 2. Just as it made the second iPad much faster than the first, the peppy A5 ensures the iPhone fast and responsive. Though the iPhone 4 rarely kept you waiting for long, here applications launch in a trice and content is downloaded fast – though this may be partly due to the improved antenna, too.

The A5 CPU has yet to show its strengths with third party apps

The CPU is swift, but its potential has yet to be fully realised yet – Apple promises performance that’s twice as fast but graphics that are seven times better. We need to wait for the improved games apps – that developers are coming up with now – to really notice the difference. Since the iPhone continues to have the one of highest pixel density ratings of any mobile, these visual treats should be worth waiting for.

The camera has also had an upgrade. It’s now 8Mp, which is good but not exceptional. The differentiators come in other forms: where the iPhone 4 had an array of four lenses, the 4S has five, promising sharper detail. The aperture is wider (f2.4) and the sensor has its wiring hidden around the back instead of on the front, where it is on most sensors. Both these aim to suck more light in through the lens, improving picture quality.

Wide mode – low resolution

And the processor is no slouch here, either, speeding up autofocus and rapidly readying the snapper for the first shot with subsequent photos captured even more quickly. Features like face recognition, auto exposure and auto focus controlled by one setting are now available. Not all of these are firsts, but they work well.

The new operating system, iOS 5, helps you get snapping more quickly by including a shortcut to the camera from the lock screen. You’ll find a more detailed exploration of the new software, which is also available on the iPhones 4 and 3GS, here.

Much has been made of Siri, the new key feature launched with the iPhone 4S. It’s not the first voice-recognition application but just as FaceTime was far from the first video calling application, Apple aims to provide the best voice interface. Siri was available in the US App Store already, but those servers no longer respond if you launch that app now. Apple liked the company so much, it bought it, as Victor Kiam used to say.

Siri originally appeared on the App Store until Apple acquired it

So does it work, and do you need it? Well, it mostly works pretty well. Its microphone does best in quiet environments and worst when it’s noisy or you’re trying to show it off to someone. Then it fails to understand what you’ve said.

In the UK, it’s limited for now because mapping and business data doesn’t extend beyond the States, though this will follow. So you can’t ask directions or distances between places as you can in America. Switching to US English and adopting your best accent works splendidly… but only if you’re in the US at the time.

Voice commands can be executed for any number of tasks

It can still do a lot. Say “Text my wife” and it will. First time, of course, you have to tell it which of your contacts is your wife – come on, it’s not psychic – but then it’ll remember. Then dictate your message and it’ll send it. Ask it to remind you to pick up the milk when you leave the office and it’ll use the phone’s location information from GPS and Wi-Fi to ping you a reminder as you walk out the door.

Siri is integrated throughout the phone’s features, and third parties can build functionality into their apps, so its potential may grow quickly. Sometimes it’s quicker to type a message, but for hands-free use when driving, say, it’s decidedly useful. Especially since you can talk to it very freely. Ask if it’s chilly tomorrow and it knows to seek out weather information and comment on whether tomorrow’s temperature qualifies as warm or cold.

The iPhone 5 could be some way off, so it's make your mind up time

Or ask it for share values and you can follow up asking it to compare two stocks. It’s highly sophisticated and though at times it seems to be a glorious, cool gimmick, this will change. One thing that hasn't changed though, is that the iPhone still doesn't offer an FM radio – something that appears on just about all competing smartphones.

So should you buy an iPhone 4S? For 3GS users, the answer is an emphatic yes. It’s less clear-cut for iPhone 4 owners, especially if you have a contract to buy your way out of. But if you suffered from poor call quality before, the 4S will restore your faith in the iPhone. Even if you didn’t, the upgraded performance and coolness of Siri and the improved camera are pretty compelling.

If you’re wary of buying the 4S in the belief that a radically different iPhone is on the horizon, the fact is, beyond Infinite Loop, nobody knows – and really, don’t believe anyone who claims otherwise.

Suggested Price: £499 (16GB), £599 (32GB), £699 (64GB)

More Info: Apple's iPhone 4S page