The KDE has a long history, all the way back to 1996. It includes some great success and innovations, but also missteps which made it the Linux desktop underdog that it is today.
I first used KDE version 3.5 which was a snappy and powerful desktop environment, with Windows-like usage paradigms. With KDE Plasma 4 they however decided to break everything by introducing a bloated and unstable desktop, which did not not bring any added value to the table. Ubuntu was now dominating the desktop Linux scene and decided their own, the Unity desktop. The KDE went through another major overhaul and introduced the Plasma 5 desktop around two years ago. As with Plasma 4, the initial release of version 5 was miserably unstable and two years of maturation on bug fixing followed. In October 2016, the Plasma 5.8 LTS (Long Term Support) was finally released.
A year ago I reported my Manjaro Linux experiences here, with the Plasma 5.6. Even then the Plasma desktop was bug ridden and crashing a lot, but now with 5.8 things finally seem to have settled. It is an LTS release, receiving critical bug fixes, but no new features, for an extended period of time.
As always, the Plasma 5.8 is extremely rich in features and customization options. The series 5 introduced a new modern flat look, which is a pleasure to look at. Thanks to Plasma 4, the KDE has a reputation of being bloated and slow, but this is not true. As I write this I am using an ancient Thinkpad X301 from late 2008 and Plasma 5 runs very good. The Qt5 framework underneath has received a lot of good development effort, partially thanks to Nokia, which had plans to utilize it in the mobile devices.
In many ways the KDE Plasma 5 is what Windows 10 should have been - a modern and extremely powerful desktop environment optimized for classic desktop usage. Major UX design decisions follow the principles introduced in the Windows 7 era, completely ignoring the range of touch enabled devices we have today. It has a classic bottom panel with start menu design, sugar coated with widgets and super smooth desktop effects.
The problem with KDE is that while they were messing with the terrible Plasma 4 and premature Plasma 5, many Linux distributions simply jumped the ship. These are your best alternatives to choose from.
Manjaro. Excellent Arch-based semi-rolling release, with XFCE and KDE as primary desktop options.
openSUSE. A classic distro that recently re-invented itself by offering two separate releases. Tumbleweed is an unstable rolling release branch, while Leap is their equivalent of LTS release, offering 36 months of support.
KDE Neon. An unstable release by the KDE team itself, that combines the latest versions of KDE components on top of the Ubuntu LTS base.
If you are planning to use KDE Plasma 5.8 as daily driver, then Manjaro or openSUSE Leap should be your choice, the latter being more conservative and sticking with the 5.8 LTS release.
The Plasma 5.8 comes at a very late hour. It remains to be seen if KDE is able to win back at least some of the lost users. While they have been messing around, the Ubuntu Unity has stabilized, Gnome Shell matured and evolved very well and MATE has emerged offering a classic desktop experience. However, if you ever thought about giving KDE another try, now is the time.