Sometimes, as we all know, looks can be deceiving. While Apple's latest iPhone doesn't look different, and may not be the kind of blockbuster people expect from the late Steve Jobs's company, it thinks different, to quote one of Apple's old ad slogans. Inside its familiar-looking body there lurks a nascent artificial-intelligence system that has to be tried to be believed.

Apple's fifth-generation iPhone, the $199 iPhone 4S, goes on sale Friday with a new operating system and a new cloud-synchronization service called iCloud. But, while its insides have been significantly improved, the phone's exterior design is identical to that of last year's iPhone 4, which Apple says is the best-selling smartphone in the world.

I've been testing the 4S for about a week to see how it differs from the previous model. I also evaluated the key features added by the new operating system, called iOS 5, including a new, free text-messaging service; deep integration with Twitter; and the ability to edit photos right on the phone.

This new software will be available as a free upgrade for owners of the iPhone 4 and the 2009-vintage iPhone 3GS, as well as for Apple's iPad tablet and its iPod Touch.

I focused on the handful of new features unique to the 4S, notably the new voice-controlled artificial-intelligence system called Siri; a brilliant new camera for stills and videos; and faster, 4G-class download speeds. The iPhone is now available from Sprint, AT&T and Verizon, but I tested the AT&T version, because it is the only one which offers the faster download speeds.

The standout feature, not available in other iPhones, or in any other phone I've seen, is Siri. It answers questions and provides information using natural language and an intelligent understanding, not just of words, but of context and colloquial phrasing. It isn't perfect, and is labeled a beta, but it has great potential and worked pretty well for me, despite some glitches.

Despite Siri, the iPhone 4S isn't a dramatic game-changer. Some new features are catch-ups to competitors. I sense Apple chose to focus more on software and cloud service than on hardware. But, in my tests, the iPhone 4S performed very well. It's a better iPhone for the same $199 entry price, at a time when some competitors are pricing their flagship smartphones starting at $299.

While some analysts and commentators were disappointed the new iPhone didn't offer an external redesign, consumers so far don't seem to care. Apple announced Monday that pre-orders for the iPhone 4S hit one million in the first 24 hours, a record that was 67% higher than the previous single-day high set by the iPhone 4 last year.

My advice is that owners of the iPhone 4 needn't rush to upgrade; they can get the new operating system. But owners of older iPhone models, or those with basic phones, will find this latest iPhone a pleasure and a good value.

Some other phones, including earlier iPhones, have rudimentary voice recognition. But Siri does much more. It offers too much to fully describe here, but it isn't a simple voice-command system. It understands a wide variety of ways to ask a question, grasps the context, and returns useful information in a friendly way, either audibly or by displaying results on the screen. It learns your voice as it goes along.

Siri can find information in Wikipedia, Yelp and Wolfram Alpha. It successfully answered when I asked it, 'Who's the president of Iran?' (though it misunderstood me the first time) and 'Who stars in 'Boardwalk Empire?' ' When I asked for a 'French restaurant in Bethesda, Maryland,' it instantly returned a list from Yelp, ranked by user reviews.

In my tests, I was able to dictate emails and text messages, even in the car over Bluetooth, without looking at the screen. Accuracy wasn't perfect -- about 20% of the time I had to try twice to get all the words correct. But, in most cases, Siri didn't make more errors than I do typing on a virtual keyboard.

Siri can read incoming text messages and let you reply via voice. If the message is about a date, Siri will even consult your calendar and tell you if you're busy at that time, and then remember to return to the message reply.

The system understands multiple, colloquial forms of a question. I asked, 'Will the weather get worse today?' and Siri answered, 'I don't think the weather is going to get worse' and displayed a weather chart. You can check stock prices, addresses, map directions and much more. It also answers in a friendly fashion, saying things like 'I'm not sure what you said, Walt.' And it has some cute answers built in. When I asked it 'What's the best phone?' it said, 'Wait . . . there are other phones?'

Siri has limitations, in addition to imperfect accuracy. It can't read the contents of email. It can't provide flight information or movie times. But Apple says it intends to link Siri to more databases. Also, Siri can reveal private data you'd rather it didn't unless you adjust your passcode permissions.

The iPhone 4S now comes with the same, dual-core processor found in the iPad 2. I didn't notice a dramatic speed gain, but the phone operated rapidly and surely, with smooth scrolling and swiping.

There is now an 8-megapixel rear camera, with a greatly improved sensor, a new five-element lens and a wider aperture. Other phones boast 8-megapixel cameras, but the 4S takes the best pictures and high-definition videos I have seen on a phone. The colors were gorgeous, everything was sharp and the camera can detect up to 10 faces. Plus, it's fast, both in taking the first shot and subsequent pictures.

Also, Apple finally has matched some competitors by allowing you to quickly get to the camera, even when the phone is locked, by just pressing the home button twice; and by letting you use the volumebutton to snap the picture. (These features are part of the free software and aren't unique to the 4S.)