Although there's a load of new features on board, iOS is still an extremely simple interface to use. If you're new to the smart phone world, you won't struggle to get to grips with it, while the hardened tech heads among you will appreciate the uncluttered aesthetic.
The new software is available now as an update for existing iPhones and iPads however, so if you have an iPhone and don't have the new interface, head into your settings to snag the update.
Power and performance
Stuffed inside the skinny metal frame of the 5S is Apple's latest A7 processor. It's a 64-bit, dual-core chip, backed up by 1GB of RAM. Before you Android fans start smirking about it not being quad-core, know that it's an extremely powerful piece of kit.
With numerous behind-the-scenes software efficiency tweaks, Apple has been able to squeeze a serious amount of power from the processor. On the Geekbench 3 benchmark test, it achieved a score of 1,419 (single-core score) and 2,567 (multi-core score), casually whupping the behind of both the Galaxy S4 (659 and 1,860) and the (904 on single-core, although its 2,623 on multi-core was higher).
Web browsing too was extremely swift, with the 5S completing the Sunspider Java benchmark test in only 403ms (Safari browser), against the S4's 1,218ms (Chrome browser) and the Z1's 738ms (Chrome browser).
Unsurprisingly then, navigating around iOS was delightfully slick, with no delay or stuttering when swiping around, pulling down the notification panel or opening apps. The camera opens considerably faster too -- something that we can thank iOS 7 for, as it sped up the loading time on the iPhone 5 and 4S too, when we put the new software on them.
In everyday use though you're not likely to notice much difference between the speed of the 5S and the speed of the iPhone 5. The 5 was already very nippy and the majority of applications you'll be playing around with don't need masses of power to run. The benefit of the extra power and 64-bit processing is that it's well set up to handle next-generation apps and games that demand more juice.
Apple has already rewritten a lot of its own software for 64-bit processing -- though they still work fine on 32-bit models -- and developers will have full access to the development kits to bring 64-bit compatibility to their own apps.
Speaking of games, the A7 processor has also had a graphics boost, although again, you might not see the improvement with current-generation games. Glossy titles like Real Racing 3, Asphalt 8 and Riptide GP 2 all played with extremely smooth frame rates.
Also inside is what Apple calls the M7 processor, which deals specifically with collecting data from the motion-tracking tech (gyroscope, accelerometer etc) without taxing the main processor. Right now, it's mainly there to help save power, but it's likely to become more important when synced with motion-tracking health accessories like the Nike FuelBand.
In theory, it'll be responsible for the data coming in from connected devices, but will take far less power to operate, thereby saving battery life. How this works in reality remains to be seen, as more apps and accessories are created to take advantage of the new tech.
The iPhone 5S boasts the same 8 megapixels it has done for the last few generations. Those of you impressed by the beefy 20-megapixel snapper in the Xperia Z1 or the 41-megapixel camera in the Nokia Lumia 1020 might not be impressed, but megapixels aren't everything. While megapixels allow for bigger images, a higher number certainly doesn't mean better-looking pictures.
Indeed, the iPhone 5 was among our favourite camera phones, so the 5S has a lot to live up to. While the number pixels haven't increased, it has a brighter f/2.2 aperture and the sensor is physically bigger, both of which should mean better low-light performance and better dynamic range.
The difference between the 5S and the 5 in my first shot, while not huge, is certainly noticeable. Exposure and colour balance are pretty similar -- both good -- but the 5S has better contrast and clarity. You'll need to look at the full screen images to see the real difference, but look closely at the brickwork on the buildings on the left -- the 5S is much sharper.
In my low-light scene, both phones are plagued with much image noise -- which is to be expected. The iPhone 5S was able to achieve a much brighter scene though, with even the darkest areas on the left and on the bottom of the trophy remaining visible.
Apple has updated the flash on the 5S, giving it a secondary flash that apparently can detect ambient light in a scene and then colour the flash accordingly to ensure accurate tones. In my first test, there was little to tell between the 5S and its predecessor. Both scenes are well lit with little image noise.
When taking shots of people, the new flash generally achieved warmer image tones than it previously managed, with less of the blue hue that's common in a lot of mobile flash photography. You'll still probably prefer to take your photos in areas when you don't need the flash, but at least when you do need to fire it up, it's not going to completely ruin the shot.
The boost in power from the A7 processor also allows the phone to take much faster bursts of photos by simply pressing and holding the shutter button. It's also able to record 720p at 120 frames per second, which will play back in slow motion. It works well and it's easy to select which parts of the video you want to be in slow-mo. Sure, it's unlikely to be the main reason you buy one, but if you're into sports like skateboarding, it could be a brilliant addition to spice up your Facebook uploads.
The existing HDR and panorama settings are on board too as well as a bunch of real-time image effects, thanks to the iOS 7 update. It's perhaps the most stripped-down camera of any top-end smart phone around, but it shoots great pictures with minimum fuss -- something that's particularly important when you want a quick snap but don't want to dive into a deep settings menu.
All iPhones sold in the UK are designed to operate on certain frequency bands. While the UK iPhone will be able to work on all UK LTE networks, it isn't physically able to roam onto all networks around the world. 4G roaming currently isn't available at all right now, regardless of what phone you have, but at such point in the (hopefully) near future that it is, you'll want to make sure you can use your swanky new mobile where you jet off to.
The UK iPhone 5S -- model number A1457 -- operates on the following bands: 2100, 1900, 1800, 850, 2600, 900 and 800 (DD) MHz.
The Bouygues network in France will support your phone, as will Free Mobile and Orange France. In Germany, E-Plus, Deutsche Telekom, Telefonica O2 Germany and Vodafone Germany all support the phone.
Italy's 3 Italy, TIM and Vodafone Italy support the required bands and Euskalfel, Jazzfel, Movistar Espana, ONO, Orange Espana, R, Vodafone Spain and Yoigo all support the UK iPhone in Spain.
In the United States things are trickier as the LTE bands used over there -- 700 and 1700MHz -- by networks like AT&T and Sprint are not supported by the UK iPhone. If you're travelling to the US, you'll still be able to use 3G networks, but if you want 4G data speeds, you'll need to borrow a different phone while you're there.
Battery life doesn't appear to have been dramatically improved over the previous model, but it did fare slightly better in the video loop test performed by my excellent colleague Scott Stein on CNET.com. The phone kept going for 11 hours which beat the iPhone 5, although not by a lot.
Scott also reckoned he could manage a full day of general use from the phone on a charge, which seems about right. As with all smart phones though, it really depends on what you do with it. I found the battery to drop significantly when playing demanding games with the brightness up, so if gaming's your thing, don't stray far from a plug.
If you're keen to preserve power, make sure to keep the screen brightness down and avoid intensive tasks like video streaming. Turning off GPS and Wi-Fi when they're not needed will also help. As a general rule though, expect to charge the phone every night when you're using it normally.
With its identical design and display, the iPhone 5S might not offer enough to tempt existing iPhone 5 owners to upgrade. If you're on older models, however, or if you're looking to take your first steps into the smart phone world and don't fancy tackling the often confusing Android, it's a superb option to consider. You'll need to save up a hefty wad of cash for it though.