I used Plasma 5 for a while and really loved the interface and the ability to customize almost anything, but what got me in the end was trying to sync settings across multiple machines. From what I can tell, there are a bunch of random files with names starting with "k" dumped in ~/.config that store most of the settings, but copying them to different machines didn't seem to work. Of course, it's not much easier to sync settings with something like Gnome, but it takes me a lot less time and effort to set up Gnome with the exact settings I like than Plasma. If I didn't switch around machines/distros a lot, I probably would have stuck with Plasma though.