Sony's Xperia Z5 premium is the flagship phone Sony has owed us for a long time. While the standard Z5 was a fairly uninspiring reissue of the brilliant Z3, the Z5 Premium packs a bigger screen with a whopping 4K resolution, which is 3,840x2,160 pixels. It's the first phone to have a 4K resolution and it's the highest resolution I've ever seen on a phone.

Cue wild applause.

But wait: Don't get too excited about seeing your photos with revolutionary levels of resolution. I personally could tell no difference in clarity on a high-resolution image between the Z5 Premium's display and that of the iPhone 6S Plus -- even though the iPhone display has half the resolution of the Sony.

Andrew Hoyle/CNET

We've debated before whether the step up from full HD to ultra HD displays is worth bothering with on small devices like phones, and that argument is even more pertinent having spent serious time with this phone. The major selling point of the Z5 Premium is its 4K display -- that's also how Sony tries to justify its whopping asking price -- but when you can't see any real difference, it's more of an empty marketing boast than a real benefit.

Fortunately, the Z5 Premium has other bragging rights, such as a meaty processor, a waterproof design and a 23-megapixel camera that puts it on par with any of today's top-end flagships. Yet it's all let down by a design so dull it could send a hyperactive toddler to sleep and a price tag somewhere north of ridiculous. Samsung's Galaxy S6 Edge costs less and looks far more stylish. It's where I'd spend my money, over the Z5 Premium.

In the UK, the Z5 Premium can be yours for an eye-watering £630, SIM-free, directly from Sony. In Australia, it'll set you back AU$1,199 but comes bundled with a digital noise-cancelling headset worth AU$79. Sony has yet to announce whether it'll be available in the US, but that UK price converts to around $960. Better start saving now.

Pin-sharp display

5.5-inch screen size

4K (3,840x2,160-pixel) resolution

The Z5 Premium's resolution equates to a massive 801 pixels per inch. That's the highest resolution I've seen on a phone, beating both iPhone 6S Plus (401ppi) and the Samsung Galaxy S6 (577ppi). On paper, that's an impressive feat, but the reality isn't quite as groundbreaking.

While you might think that cramming a truckload more pixels into a screen will make everything look sharper, I couldn't see any real difference in clarity between high-resolution images seen on the iPhone 6S Plus and the Galaxy S6 and the Z5 Premium -- even when I looked very close up at the displays to try and pick out individual pixels. The same goes for video. I showed comparison photos to several people and they agreed that the difference was so marginal, you'd never notice it was there.

Andrew Hoyle/CNET

More importantly, does it make your everyday tasks of emailing, calling and tweeting any crisper? No, absolutely not. Does it even give your mobile photography an extra level of clarity? Nope. Given the significant premium placed on it, I'd really want to notice it more.

While 4K might be worth having on a massive TV in your living room, cramming that many pixels into a small phone screen doesn't achieve anything except a higher selling price.

That's not to say this display is bad though. Far from it. It's unquestionably pin-sharp, and the colours are rich and vibrant, with great contrast too. I sometimes found it a little too vivid, with colours looking unnaturally oversaturated, so I tweaked the colour balance in the settings to find a more realistic look. It's bright too, once you go into the settings to disable the auto brightness. Oddly, you can't do this from the pull-down quick settings panel, and I found the auto mode to heavily restrict the brightness.

Tired design

154 by 76 by 7.8mm (6 by 3 by 0.3 inches)

181 grams (6.4 ounces)

Water resistant

I'm disappointed with the Z5 Premium's design. Sony has stuck rigidly to its usual Xperia look, which is beginning to look a little tired. The minimalist style, with its buttonless front, is as stark and monolithic as ever, but the squared-off sides make the phone look too blocky. The back panel is extremely glossy, and easily picks up fingerprints. I much prefer the frosted glass back of the standard Z5.

Andrew Hoyle/CNET

I once applauded the older Z phones for their slick, minimalist style, but Sony has done nothing to give its flagship phones a fresh look. Now, they're just boring, a worse fate. That's dangerous when its main rival Samsung has overhauled its top-end Galaxy S6 Edge with a gorgeous, curving design. Even LG is doing more to tickle the senses with its leather-clad G4 and textured, baby blue V10.

You'll find the Z5 Premium in black, mirrored silver or gold. The black model I reviewed is really very dull and is without doubt the last colour variation I would ever choose for this phone.

The phone has an IP68 level of water resistance, which technically allows it to withstand being submerged in water up to 1.5 metres (5 feet) in depth, for up to 30 minutes. Sony, however, no longer recommends fully submerging its phones in water. That's a real shame, as the ability to take underwater photos was one of the main reasons to buy one of the previous Z phones.

Andrew Hoyle/CNET

What this means for you is the waterproofing is there to keep the Z5 Premium safe from spilled beers, and for taking calls in the rain. The latter of which I'm pleased about, given I live in perpetually damp Britain. You'll need to firmly secure the rubberised flap over the combined nano-SIM and microSD card slot, although thankfully the 3.5mm headphone jack on top and Micro-USB port on the bottom don't need a seal to keep the water out.

Android software and Sony's skin

Android 5.1 Lollipop

Custom Sony skin

Too much pre-installed bloatware

The Z5 Premium comes with Android 5.1 Lollipop as standard, and there's no word yet on when this phone might receive an update to the latest Android 6.0 Marshmallow. I can't hold this against Sony too much, as it announced the Z5 Premium last September before Marshmallow was even available. Sony doesn't tend to roll out software updates to its phones quickly either, so I wouldn't recommend buying the Z5 Premium if you really crave your first taste of Marshmallow.

Sony's thick Android skin is one of the main reasons updates are so long in coming. I don't mind Sony's tweaks on standard Android though. It looks quite neat, it's easy to use, and you can easily sort apps in the apps tray by alphabetical order, most used or by name, which makes it easy to find the tool you're looking for.

Andrew Hoyle/CNET

Sony typically adds in quite a handful of software extras and the Z5 Premium is no exception. Beyond Sony's own calendar and email clients (the latter a baffling addition, given the phone also arrives with Google's Gmail on board), you'll find Amazon's shopping app pre-installed, as well as various PlayStation stores, Kobo's e-book app, OfficeSuite, a news app, AVG antivirus, and various other bits and bobs. It all helps the phone feel somewhat cluttered before you've even downloaded your first app. The various widgets you'll find scattered across the home screens don't help either.

Mercifully, you can uninstall many of the pre-loaded apps and I recommend you do. Before you start adding your own stuff on board, spend a bit of time clearing out anything unwanted and give yourself a blank slate to start from. You'll appreciate the reduced clutter in the long run.

Processor and cellular performance

Qualcomm Snapdragon 810 processor

3GB RAM

The beating heart of the Z5 Premium is a Qualcomm Snapdragon 810 processor, with 3GB of RAM. As that's exactly the same setup as you'll get inside the Xperia Z5, it's no surprise it gave almost identical results on my benchmark tests.

On the Geekbench 3 test, it achieved 609 (single-core) and 2,861 (multi-core), while the standard Z5 achieved 610 and 2,926 respectively. Similarly, the Premium racked up an impressive 26,180 on the 3DMark Ice Storm: Unlimited graphics test, while the standard Z5 achieved 26,887. Clearly then, there's no difference in performance between the two sizes.