Four reasons you should update to iOS 9 (and one reason you shouldn't)

Apple's next big update to the iPhone's software is here. iOS 9, announced back at WWDC in June, includes plenty of features that we think make updating soon worth your while – and one big reason why we think you should hold back.

1. Multiple apps on screen

Let's get this straight: iOS has always been able to multitask, in the strict meaning of the word. It is, after all, based on OS X. What users haven't really be able to do, though, is run two applications on-screen at the same time.

2. Proactive Siri

In iOS 9, Siri is getting smarter. In addition to all the old features, when you slide left from the homescreen Siri will suggest apps you might want to use and people you might be interested in contacting, based on your previous behaviour.

It will also suggest news for you. It's not quite the anticipatory delight that's offered by Google Now, but it's pretty good and a definite step in the right direction.

3. Smaller apps

For anyone struggling with a 16GB device (why does Apple insist on selling these?) there's app-thinning. This means that – rather than downloading the code and assets for every single iOS device, including (say) stuff only used on the iPad on an iPhone – you only get what your device needs.

This is likely to save a lot of space, especially with graphics-heavy apps like games.

4. More power

Every update seems to reduce your battery life. Not iOS 9. In fact, Apple has introduced some additional power-saving options.

As well as a new power-saving mode designed to eek out battery life when it gets low, devices will now use their sensors to work out whether your device is, for example, face down on a table – and then not turn on the screen when a notification arrives.

And one reason why you shouldn't upgrade…

In a word: stability. We've been testing iOS 9 for quite some time and our opinions are mixed. Some of the team have had no issues beyond the kind of occasional crash that you'd expect from pre-release software.

Others have been seen swearing at their iPads every five minutes as yet another application broke or the machine locked up entirely. With every new release, there's always a short period when it's more flaky than what replaced it.

If you're risk-averse, waiting a couple of weeks for the inevitable iOS 9.0.5 release might be a good idea.