I've been using OneUI and Android Pie on my brand new S8+ for a couple of weeks, and here is what I think of it so far. I am aware that that feedback is completely subjective, you might not think the same.
- General opinion I haven't used Android 8.x at all, but I was a power user of Kitkat and Lollipop. Android 9 feels like a big updrade AND a massive downgrade at the same time. It is very smooth, gives me a ton of options, that's what I was expecting on a brand new 2 years-old flagship anyway.
What I didn't expect, though, is that by upgrading to Android Pie + One UI, you lose a ton of stuff as well. Starting off with the terrible, terrible multitasking. Why would you have only one app on the multitasking screen at a time? Why don't you see the icon for dual window anymore? It is so painful to navigate through that screen when it was so well-done on previous versions of Android... Is this iPhone or Android?! Another thing I noticed (like everyone) is the lack of an option to hide the navbar. Why are we forced to use either crappy gestures or see the navbar full-time (especially when the icons are terrible, coming back to this later)?! The UI offers an easier way to use that giant screen, sure; you can finally remap the Bixby button natively, sure; the system-wide night mode is very handy, sure, but... C'mon, that phone doesn't feel special anymore.
That, and the battery doesn't last as long as it should, so I'm assuming it's because of the new Android.
- Camera app What's that? A camera app that has a dedicated button to change what camera is used, but not one that allows you to take videos? AR Emojis? Bixby vision?! I mean, I get it Samsung, you're trying to please more the young users, but I'd prefer, y'know... having the choice to see that...
Also, the Pro Mode is a huge, HUGE downgrade from my LG G4. There's so few options, I can't take a correct picture (when taking photos is the reason I bought that phone in the first place). And having to search through pages of useless modes doesn't help, when I could get to it in two taps on my G4.
- One UI itself I'm not a big fan of the new default app icons - that's kinda subjective but I feel like they could have been drawn by a kid. I'm NOT a fan at all of the new UI icons - the navbar in particular. It looked futuristic, it looks just bad now. And the fact that you can't make it transparent and hide it natively is such a hassle. I am not using the default Samsung Experience launcher, so I can't really speak about that, but the lockscreen is now terrible - it looks washed out. Apart from that, I haven't much to say - it feels like a regular Android experience to me.
TL;DR : I'm rolling back to android 8 and don't recommend upgrading until every problem here has been adressed, but I feel like they will never be and that's a shame.