Previous (9/8): As far as looks are concerned, the Samsung Galaxy S8 and Sony Xperia XZ Premium could not be any more different from each other. Even with that difference, however, both devices feature HDR-capable displays, and you can now include YouTube as another app that takes advantage of their displays.
When viewing HDR content through YouTube on either device, you should now see HDR when you try to manually change the video resolution. Speaking of which, the maximum resolution currently sits at 1080p, even when playing 4K content. Even so, the content should still look nice and sharp on the Galaxy S8’s, Galaxy S8 Plus’, and Xperia XZ Premium’s displays.
As for whether HDR is worth having on phones, it is a bit of a toss-up. HDR content tends to be brighter with better contrast and a wider color gamut, all of which are pleasing to the eyes. However, because multiple phones use different HDR standards, things can get a bit confusing when it comes to support.
For example, Netflix HDR playback is available on the Galaxy Note 8, LG V30, LG G6, Xperia XZ1, and Xperia XZ Premium, but not the Galaxy S8, Galaxy S8 Plus, and Galaxy Tab S3. Samsung teamed up with Amazon to announce an HDR standard, but Amazon Prime Video currently does not offer HDR playback on smartphones. Google Play Movies & TV also offers HDR content, but you can only watch it on a supported TV monitor, not a smartphone display.
In other words, there does not yet appear to be universal support for HDR because there are so many HDR standards and because different companies opt for different standards. Even so, it should only be a matter of time until YouTube HDR playback arrives on more HDR-capable smartphones.