Smart Stay isn't a new feature for the Galaxy S4, but it's a better implementation than we saw on the Galaxy S3 last year. Back then the result was a little patchy, and also contributed to some dodgy auto-brightness levels.
In case you don't know, Smart Stay is another of Samsung's eye-tracking technologies, one that can tell when you're looking at the screen and won't dim or put it into sleep mode as a result.
This time around it's nearly flawless at checking out when your eyes are looking at the screen, although when it does get it wrong and things begin to dim there's no way to save it (despite us blinking and flashing our eyes at the display in the vain hope the S4 might recognise the effort.
Of course, you could just tap the screen with your finger - but come on, this isn't 2011.
For all the brilliance of Smart Stay, the world's eyes are on Smart Scroll (sorry). This technology was designed to also monitor your eyes, but when it notes you're looking at the phone the Galaxy S4 will enable you to tilt the handset back and forth to move the text or email you're trying to read up and down the screen.
Well, this is what Samsung said at the Galaxy S4 launch, but it turns out that there's another, more cool, trick at work here: you can hold the phone steady and tilt your head up and down to achieve the same thing.
You have to make a pretty strong movement with your head to make this function work, but when it does it's pretty cool indeed and one of the 'down the pub' moments that will make people sit up and take notice.
However, and this is a big one: because the feature isn't perfect, we can't say it's a useful way to navigate around the screen. It's cool that you can choose to either make the screen scroll using your head or tilting the screen, and then you can select the speed of the scrolling too.
You can also decide whether to have the annoying eye icon pop up on the screen to alert you that the Smart Scroll function is working. So while it doesn't really work every time and sometimes messes up, it's cool to have - although we still have the issue of using your finger being a more useful way of scrolling around a screen.
With something like Smart Scroll it has to be flawless to be considered a useful part of life, and would have mitigated the issues we found with the actual need for it. But as it's slightly buggy and dependent on a number of factors (such as lighting conditions) we just found that after a few days it got turned off - and we fear that's the way it would stay for most for the next two years.
One of the new features of the Samsung Galaxy S4 is Air View - well, it's not new per se as we've seen it on the Galaxy Note 2 already and has been supercharged with the Galaxy Note 3. But where that device needed the S Pen to work, the S4 only needs a finger, which you hover over certain items to see inside without opening.
Samsung has imbued a number of applications with this functionality, but in truth only a few really need it. For instance, being able to see which speed dials are assigned to which number is really useful, as otherwise you'd just have to press and find out.
Less useful are things like video scrolling, where you can flick through the video using the timeline bar without having to disturb the main action. While this is a useful feature, there's not a lot of point to having to hover the finger over the screen to achieve it when you can just slide your finger on the screen, which is a much easier way of doing things.
What's nice is that in the Setting menu you can choose what Air View is used for, so that means you can turn it on for emails and messages, but off for video previews and magnifying the web.
The only problem is that you'll have to get used to holding your finger further from the screen, as the screen is a sensitive little monkey. You'll be scrolling through an email and suddenly you'll see items flashing up all over the place, meaning you'll want to turn off the function. We'd say it's worth putting in the effort to get better with it, but we'd rather Samsung got even more granular here to let us turn off Air View in email, where it's easier to trigger and less useful, but keep it in calendar, where it's a blooming godsend.
That said, it's still probably one of the best innovations used on the Galaxy S4, and we're glad it's been added in.
Where Air View was useful, here we come to one of the things we thought was the least practical things on the phone in our hands on preview: Air gestures. It's designed to let you simply wipe your hand over the front of the phone without touching it and means you can skip tracks, move between photos and answer calls without touching the phone.
We'll say that the latter functionality is good, but only when you're in a hands free situation, such as the car. There you don't want to be having to root around for the 'Call accept' function when you're supposed to have two hands on the wheel, where a simple wipe to answer is really cool.
We've got nothing against the option of doing things this way, and it's not a lot more accurate with the new software update. This means that there's no more (well, nearly) missed gestures, and it won't activate when we don't want it to, which is another real bugbear we found at the start.
Other uses, such as moving between tabs in the internet browser and moving app icons around, are cool and could be useful in very niche situations... but it's still not really better than just touching the screen and doing it without worry.
There are some more instances where it's really, really useful – you can wipe over the screen and flick through PDF pages, scroll through web pages a large jump at a time or flip between tabs.
It's a clever system, but like we said, it's not useful enough – it's cool to do, but takes a little more effort than it should to be a natural flowing part of your phone use.
Like so many new features on the Galaxy S4, a little annoyance means you'll turn them off, and it's unlikely you'll ever turn them on again, which makes us feel bad for all those engineers that were asked to come up with all this innovation.