My experience:
Installation: very fast (well, I got fiber/gig-e earlier this month) with cli / urpmi. I got rid of orphans too.
I noticed that the kernel had not been updated, manually added the mga6 desktop kernel.
Booting: grub2 had added the mga6 kernel as default.
Booting was similarly fast on Mageia 5, all fine. About 15 secs from grub.
Login screen: no icons for users (don't mind, don't care), looks slick. Options to log in include icewm and plasma. I assume that plasma is what I need (KDE user since forever).
Logging in into the main account (my wife's - my stuff is on a headless server living in a vnc session that I can connect to from about anywhere, including my smartphone over vpn): slow to log in (25 to 30 seconds), are you kidding me?
Next, with the new plasma, all settings are "gone".
Okay, no biggie, I'll have to redo them.
First things first: everything is soo tiny on my 24" 4k screen, so I configure to set the dpi to 184 (in a second approach 142 suits me better), change icon sizes, log off, log on (incl. waiting), ok, now I can see what I'm doing.
Then, I figured I better put back the various backdrops and such that my wife had.
We had the marble desktop globe as a background for the first of 4 virtual desktops, and then specific images for the other 3 (organised in 4x1 virtual desktop arrangement).
First, the default seems to be that there is only one desktop - so the pager (which was there) actually showed nothing, since there was nothing to page.
Then (after searching) I find that not only can we not have Marble as a globe (it was possible to zoom in or out and keep the setting/view as the regular desktop background), it is no longer possible to have different backgrounds for different virtual desktops. KDE devs seem to have taken a hint from the Gnome guys or so..?
Sure, life will go on, but activities are really not the same as virtual desktops, so this regression makes me sad...
Anyway, the next thing I found that's really not ok, is that suspend to RAM doesn't work.
Basically, it wakes up to the final screen (usually black with mouse pointer), no more response after that, caps lock won't react, basically dead...
I tried with the old mga5 kernel as well, no go.
I managed once with CLI "pm-suspend", a second time it failed and got stuck in the "usual" way.
Not very cool, considering that the login is much longer as well...
I did try to figure that one out, a new account does log on within about 10 secs.
So I moved all ~/.config stuff aside and tried again, it did seem a bit faster on the main account.
After fiddling with desktop effects, it got slower again.
Desktop effects (notably grid view of virtual desktops) got the pc to lock up hard a few times. Uncool.
My regression list:
broken Suspend to RAM
broken Suspend to Disk
no more individual wallpapers per virtual desktop
hard lockups when playing with desktop effects that have been problem-free since Mandriva times
no more gapless playback (for concept albums like Misplaced Childhood) - missing in action since about when PulseAudio took over; I know for sure that about 15 years ago it was working fine (after following guides to set things up properly), and at some point, it suddenly wasn't...
All in all, this machine is now not as functional as it was before.