I upgraded from a 6 Pro because I shattered my 6 Pro's screen a couple of months ago.
Overall I like the upgrade. A lot of it is very similar - but the most noticeable improvement to me is the flat screen vs. the curved screen from the 6 Pro. I also appreciate the 5x optical zoom over the 6's 4x. It's also noticeably better at macro shots - did the 6 pro even have a macro lens?
I don't know if the 6 Pro had an OLED screen or if the 8 Pro just does a better job showing it off, but it does a little lighting effect with my photos that make the per-pixel lighting noticeable in a way I never noticed on the 6 Pro.
The one major downgrade I've noticed, which I think also affects the 6 Pro since it's software, is that they removed the ability to take a photosphere from the camera app. That's a major disappointment since I was a big fan of that feature, and I'm on the lookout for a competent replacement app for that functionality.
Also a very minor annoying change that I know affects the 6 Pro too: The Calendar widget can no longer be shrunken to be only 3 tall. This is particularly annoying because I had it that size on my 6 Pro, and noticed I couldn't on my 8 Pro (before trading it in) - so I made it 4 tall on my 6 Pro to test out, and it never let me put it back to 3 tall again. Seems like a pointless change that gives me less space.
Pro tip: If your trade-in phone has a cracked screen, just lie about it. They were offering $200 for a working 6 Pro with cracked screen or $400 for one without a cracked screen. My crack was pretty noticeable but it passed inspection anyways and I got the $400. If they do notice it you'll just get bumped down to the $200 you'd have gotten with honesty anyways. Also sign up for the $2.50 trial of Google One, which gets you 10% back in store credit for Google Store purchases - that $2.50 one month trial got me my phone case, screen protector, and camera protector for free after the credit from the actual phone purchase.
I feel like I was pretty rambling here. You can ask about specifics if you have any.