TL;DR: installed Manjaro 2015/12 KDE on my notebook,
some hours swearing, happily switched back to Kubuntu.
Installation "setup" script (text-mode) pretends to not to see certain errors: you will figure out later, when it won't boot. Examples: EFI boot setup did not work. Gummiboot appears to be absent. Pacman databases creation fails because network connection was silently ignored (my b43 "non-free driver" was ignored, the tg3 ethernet chipset failed to setup at first: had to manually rmmod/modprobe/ifconfig/route it). Funny, the btrfs aborted refusing to create a filesystem in a partition already btrfs-formatted (Manjaro "setup" didn't notice that I was repartitioning; even if wiped with dd if=/dev/zero the same-size partitions left the data there). After Grub issues, I switched to SysLinux and finally Manjaro boots.
Man, KDE5 Plasma sucks. Or, at least, it's the Manjaro version that sucks. After changing a number of desktop settings (theme, colors, and so on), the screen started flickering. Logout, login, same problem. Reboot. After a minute, same problem. Arrrrgh. Reinstall everything from zero. Modified fonts for Konsole and Yakuake: the "intense" colored text goofed like a drunk, UTF8 support was lacking, and yeah, only Terminus and Oxygen Mono fonts worked. Selected "en_US" language with "it_IT" date/money/collate support, and the control center and some widgets borked in an Itanglish chaos. And that ugly Kwallet, requiring me to type (again!) a password before it can connect wifi at login.
Octopi failed to update the system. Entered a command-line shell and started sudo pacman -Syu and got a weird bunch of write errors. The root directory was full: the 6400 Mb default suggested by the "setup" was barely sufficient to install the KDE without updates. Decided that reformatting everything was faster than btrfs partition resizing. The home directory in a separate partition only makes sense if you want to encrypt it.
Widgets, what a PITA. Some just don't work (quick share is missing a "plasmoid", input method silently ignores clicks); others are either buggy or amazingly amateurish (network monitor, thermal monitor, date/time... sir, why can't I have a "fri 5 feb 16:02:22" there?), other are too byzantine (audio volume, spectacle screenshot), most of them would work a little better if you double the height of the panel.
The flickering error appeared again. Man, it's time to switch back to Kubuntu.