As it's getting quiet, I'll give my 2 cen... p.
After having gotten used to the OnePlus 7 Pro 5G and the excellent OxygenOS, it is strange to get back to OneUI. Also it missed this version, I now miss a few thing from OxygenOS, like the last gestures on the side of the screen, so convenient for one handed use, the excellent fingerprint sensor, etc.
The transfer and reinstallation took me just two hours, like someone said earlier. Smart switch was great for that, bringing all the pictures, videos, accounts with passwords, apps, even those proprietary of OnePlus. 😄
Lots of bloatware to remove later, a little bit of tweaking there and there.
I did some pretty heavy use at work with the upgrade as well. The phone got quite hot but the room where I was was hot as well. Still, I take it this is the downside of the Exynos chipset compared to the Snapdragon in the OnePlus 7 Pro 5G.
The camera is obviously much better (108MP pictures packs lots of details, good for cropping), the screen looks good and the phone works well, apart for one little thing that bugged me. The 4G signal seeemed very poor today at work, so I wonder if it's a one off or if the connectivity isn't as good as what the OnePlus offers. I haven't much researched the subject so I don't really know and it's too early to be certain.
Battery wise, it's really good. I run the screen in Quad HD instead of 120Hz because I like to watch videos in 1440p better than in 1080p - hopefully Samsung will release an option for QuadHD+120Hz. No comparison for the OnePlus which was doing OK last week but got its battery killed by the last upgrade(I could finish the day on the 7 Pro with 20-40% of battery left but after the update I had to be careful to just not empty it before going home).
In the end, my main reason to move to the S20 Ultra was for the camera. I had a Canon 5D3 with expensive L lenses in the past (sold for a motorcycle), so I now rely on my phone to take pictures and here, I'm not disappointed. It's a new sensor, it'll take some time for Samsung to polish it the way they did on the Note 10 with the previous generation but it's already prettyy good and fun.
