In 2003/2004 I used Linux desktop at my job. I used Redhat 8 and 9 with KDE. It was usable. Ten years later, default KDE on Ubuntu 14.04 is barely usable - too many annoying things. Plasma 5.3 looked promising, but "not there yet". Unity is at least stable, but completely unconfigurable and I *hate* window buttons being on right. Also, selecting a window from the panel is completely annoying as you have to click on the panel, and then to move mouse to select screen - complete waste of time. I now use Cinnamon, and it is ok, except that I had to tweak UI via changing CSS file, and that power off window does not show "ok" button, only "cancel". And I have problem setting Alt-Shift for changing keyboards. And probably many more things but I did not have a chance to notice them yet.
From my perspective it looks as if Linux desktop is going backwards. I believe that problem is that there is no big companies behind it, like it was Redhat it its days. Yes, there is Cannonical, but it seems more interesting in pushing their political agenda than in desktop itself.