As I did with the advent of KDE 4, and as is evident from the approach that a lot of Gentoo users are taking, I decided to try KDE 5 by running someone else's live-CD version. I got the Slackware version (no systemd, you know). For some reason it wouldn't run in a VM for me, so I booted it from a USB stick on bare metal.
Instant impression: that dorky flat icon theme that is so much in fashion nowadays. (The world today has not only too many lawyers, but also too many graphic designers.) Fortunately, it's not hard to switch back to Oxygen icons. Fixing the panel icons will take a bit more work--but I already have to do the same kind of thing in KDE4.
The very first thing I did was switch to the older, flyout-style menu. There's no separate context-menu selection to switch to the classic menu as in KDE 4 but rather a choice to choose among multiple alternatives. Here you look for Application Menu. (One of the nice things in KDE 5 is that the context menus of many of the widgets offer to let you choose a different implementation of the widget.) The new Application Menu is almost like the KDE3-like menu in KDE 4 except that 1) there is a column of icons that let you choose certain items quickly--but I don't see how you can configure it, 2) there is an entry field for a search (I guess to make it more like Windows), 3) no Run Command entry, 4) no Favorites entry (maybe that's what #1 is supposed to be--I never figured out how to configure Favorites either), 5) Leave is now called Power / Session (I like "Leave" better), 6) now you have to hover over Recent Applications to see the recent applications, and 6) there is a new Recent Documents item. Other than that, it's

just like the KDE 4 version.
I don't think that any of those changes in the menu are improvements, but maybe that's just me.
Bah! on their "cleaned-up appearance". It's now harder to find things. In general, you have to dig farther into menus to find the selections you need. On the other hand, it's not awfully easy in KDE 4 either.
Now the worst omen I encountered. This is where I started looking to the KDE bugzilla but the applicable search term was wayy to broad: "kde plasma frameworks 5 about kde no version". I refer to what you get when you select Help -> About KDE: it doesn't show you the version. The
version! You know, the very reason
why you want to go to the About KDE selection in the first place. The per-application versions of this (for example, About Konsole) will let you find the version, but you have to do another click to get to it. I don't know Slackware. How am I supposed to figure out what version of KDE I have? I came to the Gentoo Forums hoping I could find something about this because this is a thing I couldn't find in the KDE bugzilla.
Clearly, there are too many graphic designers in the world and nearly enough usability experts. This might finally be the break that makes me leave KDE.