I have the 65" and I'd give it a B+, which at the price point is a solid grade.
Motion: I usually use Blur: 6 / Jutter: 2 -- Anything more than that and my set can stutter. I really hope Samsung can fix this with a firmware update because the set has enough horsepower where it shouldn't be an issue. Also, someone mentioned this earlier, but set Sharpness to "0" and you'll have a better experience with motion.
Picture quality: Amazing. I just spent a couple of days calibrating with my eyes and I'm beyond satisfied. Infinity War 4k HDR looks _amazing_.
Dimming: Atrocious, again, I hope Samsung can address this with firmware, or at least give me the option to turn it off. I honestly can't tell the difference between "Low" and "Standard." Why not just have "High, Low, Off"?
Popping: I understand wanting to provide a theater experience, but the popcorn machine is a bit much. It lasts maybe 5-10 minutes of turning the set on, not a big deal for me. My PS4 pro does the same thing after I turn it off. Actually my TCL 5 series in my bedroom does the same thing to a lesser degree. Climate+heat issue maybe? No idea.
UI: At least on my set the UI is QUICK (I use Cloudflare's DNS on my router, so maybe try that for anyone having issues with latency, it's quicker than Google's imo). Also, disable the default "channel" or I believe it's "lock" in Apps, that should help as well.
Refresh: This is where this TV shines (besides the PQ). 1080p @ 120 Hz is sick for PC gaming and I don't think the set gets the love it deserves because not everyone is going to use this, but if you do, you're in for a treat.
Overall, the TV has its faults, some of which will hopefully be addressed through updates, but after 5-10 minutes of it being on the popping goes away and I'm left with an AMAZING picture.