This will not be your normal, run-of-the-mill review. In normal reviews, the product’s fully cooked, to the point where you can fairly evaluate every aspect of what the New Thing does.

Since this is a review of iOS 9, we’ll all need to wait a little while: it now transpires that we won’t be able to play with its star feature - 3D Touch - until the new iPhone 6s and 6s Plus reach UK stores on September 26.

For now, let’s call this review a work in progress: be sure that we’ll update it as soon as a 6s lands in the Stuff offices in a few weeks. That said, we have spent a full week testing it now on our iPhone 6 Plus 64GB. That’s enough time to know whether iOS 9 is a serious leap forward for the smartphone experience, or conservative refinement of an already successful OS. There’s certainly enough new stuff in iOS 9 to warrant anticipation among Apple followers - albeit some of it will be hidden under the hood for owners of pre-iPhone 6s devices.

Apple claims to have pulled off a trick that evades most upgrades in real-world usage: improved battery life (up to an hour more, they say, from a single charge) and snappier performance.

Even if you’re not upgrading to the new hardware, you’ll see above-the-hood changes from the iOS 9 update - the new Siri Suggests search screen (to the left of the main home screen, prompting parallels with Google Now), a new default font, hefty updates to several Apple apps and the arrival of Apple News.

The experience: iOS learns the search game

Truth is, most people upgrading to iOS 9 won’t notice a ton of difference. You will still, mostly, have the iOS interface you’ve known, loved - and occasionally been irritated by - for years.

No, you will not be able to move the icons to any point on any home screen (as you can on Android). No, you will not be able to pop widgets on to any point on any screen (as you can Android). It was iOS, and it’s still iOS.

But unlike previous iterations, there’s one enormous difference. Swipe to the left from your normal home screen, and there’s a Siri Suggestions screen. By Apple standards, this is a revolution on the scale of the fall of the Berlin Wall. A whole new screen. And you can’t put icons on to it. Lord save us.

Time will tell just how useful the world’s iPhone lovers find Siri Suggestions. The good news is that it’s nicely designed, and works. If you want to find something fast, type or talk and it finds it (as long as it’s an Apple native app, or a third-party app that has been updated for iOS 9 compatibility). And if you want a shortcut to a commonly used contact or app, it’s right there on the new screen. 

Apple claims that the intelligence built into iOS 9 will learn your habits, and change the apps and contacts shown on the Suggestions screen to meet your needs. I can only guess that a week isn’t long enough to have trained the new system, or that my app usage is narrow and entirely predictable - either way, those apps have yet to change enough to suggest some magical predictive brain behind the scenes.

Elsewhere, you’ll notice that your beloved iPhone is subtly prettier - thanks to the new San Francisco font and some updated app icons - and a little easier to use. You can now switch back to a previous app, thanks to a new breadcrumb that appears at the top of the screen - no more closing and opening.

So for anyone upgrading their iPhone 5 or 6, they’ll get one new screen, a ton of smart refinements and one brand new app. But the real change to the iOS experience is reserved for buyers of the new iPhone 6s and 6s Plus.

You can’t accuse Apple of hype when it says that 3D Touch is an innovation that’s every bit as profound for touchscreen devices as the original tap, pinch and swipe. We’ve already had a quick hands on with 3D Touch on an iPhone 6s, and it will bring new, better ways of working with your content. And that’s not a claim that many mobile operating systems can make with a straight face, at least not in recent years.

Apple boasts that people are switching (back, probably) from Android to iOS, and has even launched an app to make the transition easier (which was duly trolled to within an inch of its life by the world’s Android fanboys). We can see why.

iOS may be a prison compared to Android - it’s impossible to customise the interface in any meaningful way, and the UI is stubbornly unwilling to bend with how you’re most likely to use your device every day - but it’s a brilliant form of incarceration.

Performance: enough to make you upgrade to a 6s

Apple hit a sweet spot with iOS 8.4.1. The latest update had ironed out just about every crease in iOS 8, to the point where our 64GB iPhone 6 Plus was about as near to perfect as any smartphone could be: two days' battery life in normal use, and an interface that was completely lag free. Heaven.

All of which makes the iOS 9 release something of a risk, made greater by Apple’s aggressive claims of improved performance and longer battery life (pride before the fall, and all that). But before we make a grand pronouncement on how successful Apple has been, we need to give a major health warning. The world’s app developers won’t update for iOS 9 compatibility for several weeks yet, so increasing the chance in the meantime of slowdowns, crashes and mysterious battery drain.

At least if the last week is anything to go by, the iOS 9 upgrade walks a fine line. On one hand, it’s snappy enough to make most iPhone 6 owners glad they made the upgrade (and the same can be said for the iPhone 5s, based on 15 minutes with an upgraded example). On the other, there’s a few slowdowns that may just be enough to nudge you into buying a 6s or 6s Plus.

In particular, the new multitasking screen is a slight disappointment on our 6 Plus. iOS 9 replaces the inefficient app-per-screen view of iOS 8 with a stacked deck of minimised app cards to horizontally swipe through (not dissimilar to the Android Lollipop multi-tasking view, but turned 90 degrees).

Of course, there’s every chance that yet-to-be-optimised third-party apps were slowing down the animation as we swiped through the new deck. But even when we closed all third-party apps and flicked through a deck of Apple's native apps, the slight jerkiness was still there.

There’s the same subtle slowdown when you pull down from a home screen to start a search - the transition looks like it’s running at a lower frame-rate than you expect from a leading mobile OS. Why do we get the feeling that it will be as smooth as butter on the A9-equipped 6s or 6s Plus? And why do we feel so cynical for thinking that the slowdown is deliberate? Perhaps it's because our immediate reaction when encountering the small lags was to start checking the price of a 64GB 6s Plus.

That said, the rest of iOS 9 is as smooth as it was in iOS 8.4: apps open and load quickly, and the swipe between home screens is instantaneous. And that’s a state of affairs that should only improve as app developers update for iOS 9.

Battery life appears just as good as with iOS 8, although it’s obviously too early to tell if it provides the improvement of up to an hour that Apple claims.

Our 6 Plus would end a normal day’s use at 65-70%, and a heavy day at 45-50%. A week with the 6 Plus running iOS 9 seems to be following the same pattern. Which, by any standard in the smartphone world, is bloody excellent. Avoid playing Riptide for five hours solid, or leaving every one of your installed apps to full sync and with notifications on, and you should get two days’ usage without even trying.

If the new 6s Plus has the stamina of its forebear, you’ll never need to use iOS 9's new Low Power Mode mode (although we’ll understand it coming as a boon to iPhone 6 owners, with its smaller battery).