Thought I’d start a thread for this. My first impression was meh. My second impression is I wow, I love it. I turned off the top bar buttons, put the new built in dock on the left with no favourites always hidden (to use the application and workspace buttons)
Then I run plank at the bottom which is business as usual for me.
I only run a few gnome tweaks and the all worked flawlessly. These are transparent top panel, Caffeine and the audio selector one.
Very happy with this. 10/10 System 76.
My first impression of Pop 21.04 was also 
Maybe I should also reinstall and try and tweak it a little more but I tend to rate distributions based on their defaults. COSMIC feels like a step in the wrong direction rather than backwards — 3/5
I did the upgrade yesterday so it’s a little early. Changes are indeed not looking mind blowing but I like the dock, I put it on the right side (hidden). I’ll now have to look for the tweaks I was running before but so far it still comfortable to me.
Just upgraded the DarterPro. Had a little choppy experience as I had some PPA issues nothing impossible.
I used Tim Morton’s Bible Analyzer software and immediately noticed a few things that are not working that stuttered in the previous Pop.
I noticed by default, the newest version of LibreOffice is featured. (very nice!!!)
Had to hunt around to find the Appearance section in the settings app. I used to just hit ‘super’ and type “appearance”… now a ‘quick launcher’ pops up and doesn’t know what I’m searching for. Not sure if this is going to stay around long. 
(Seems kind of redundant of what we had before, but less featureful)
A good couple of shortcuts seem to have changed.
Also: I noticed that the bible-kjv package ( a CLI Bible search tool) has started misbehaving under 21.04. I can still lookup references, but I can no longer search for strings. Odd, because it works on every other non-Pop-21.04 system I own, and worked fine before the upgrade. Its an old package, so I can’t help wondering if whatever command it uses under the hood to search the concordance, or maybe something else. But so far, this is the only package I’ve run into during regular routine use that I’ve had issues with.
This might take a bit of getting used to.
Overall, I approve.
I had 20.10 installed on my System76 Lemur6 laptop. I updated through the settings app Thursday evening. I like it better than 20.10 but it’s not knocking my socks off or anything. I’m just not a gnome fan TBH. Pop!_OS is installed on that laptop because it’s about the only OS that will run on that laptop without random shutdowns.
I’m guessing they didn’t go to 3.40 because they’re still using Ubuntu as a base.