Ranked in these QuestionsQuestion Ranking
Pro Highly flexible
There are many customization options and possibilities to tweak the desktop, including widgets.
Pro KDE is an evolution on the classic desktop model
KDE is a great evolution on the classic Win95/XFCE approach. It's moving in innovative directions while respecting the classic metaphors.
Pro Has a file manager that provides a good balance between power and simplicity
The included file manager provides several icon, list and detail views to choose from along with features such as tabs, bookmarks, tagging, previews and metadata, network file access, bluetooth file transfers to/from devices and excellent removable storage integration while remaining fast and easy to use.
Pro Looks beautiful
The design of the three built-in desktop themes; Air, Breeze, and Oxygen, are very beautiful to some.
Pro Comes with a suite of powerful applications
Plasma Desktop generally comes packaged with a full set of applications to get users started, including a file manager (Dolphin), advanced file manager and browser (Konqueror), image and document viewers (Gwenview, Okular), the Calligra office suite, CD and DVD authoring (K3b), and dozens more. The desktop can be installed and used without these applications, but they add significant value for many people.
Pro Adheres to standards
Standards adherence allows for interoperability and shared technology for X Window System desktops, with similar Wayland support being worked on. Applications not written with Plasma in mind work very well in Plasma as a result. The development team has also been instrumental in standard creation and adoption such as NETWM, X11 clipboard, icon themes, mimetype handling, application menu standardization, system tray protocols and notifications and more.
Pro Keyboard friendly
Nearly all actions can be driven with keyboard commands. Window management, including effects such as desktop overviews, can be triggered with a keyboard control (or mouse gesture) and some even support filtering results (such as windows shown) by typing. The KRunner tool (default keybinding: Alt+F2 or Alt+Space) provides searching local files, online sources, unit conversions, math and more all from a keyboard driven interface.
Pro Integrated advanced search
Plasma Desktop comes with an integration search system that makes it easy to find local files, emails, contacts, events and more. The file manager supports tagging and rating files as well as full-content searching and the KRunner command window and the Milou desktop widget makes searching for files, emails, applications and other content by name, subject, category, tag, fulltext, etc. very simple. It does this with essentially no noticeable interference with day-to-day usage of the computer, thanks to the scheduling built into the backend system (Baloo).
Pro Many coherent applications
What make plasma so nice is the galaxy of apps, sharing same look and feel, configuration and behaviour. This helps with making for a uniform looking desktop.
Pro Multi-Device "Convergence"
Plasma Desktop provides seamless "zero config" integration of your Android device with your laptop and desktop machines via KDE Connect. Phone calls, SMS messages, cross-device copy and paste, media remote control, cursor control and more are supported.
The technology that Plasma Desktop is built on, simply called "Plasma", also provides interfaces for phones, tablets, netbooks, and media centers in addition to the desktop. These additional interfaces use the same underlying frameworks and therefore work well together and have a unified feel to them. They also support a common set of applications across them which adapt to the input methods and screen sizes.
Pro Very customizable
One of the best aspects of KDE is that it gives you Lego-like tools called widgets. You can combine the widgets in the way that better fits you and get a Mac OS desktop layout, a Gnome 3 desktop layout, a mobile device desktop layout or a completely new desktop layout that works for you.
Pro Fast and efficient
Looks great! Dolphin file manager is without a doubt the best fully functional and easy to use and multitask with.
Pro Enjoyable to use
Thanks to looking awesome, and being customizable and flexible.
Pro Activities what multiple desktops needed
You can really separate work ambient from personal ambient.
Pro Not based on GTK
A lot of users don't really like GTK's style and way of doing things.
Con Stability problems
Under certain conditions, most of KDE's components can be highly sensitive to race conditions, which leads to KDE applications frequently crashing, and, on rare occasion, kdeinit itself locking up.
Con Perceived clunkyness and slowness
Emphasis on perceived. It's a myth from the days when SSDs, gigabytes of ram and cpus above 1GHz and more than one core were a fantasy. On anything semi modern (i5 2500k, 8 gb memory and 256 gb ssd is total overkill and that's a 5 year old system) it's as fast as anything.
Con Add-on installation can be tricky.
Adding themes and widgets can be tricky.
Con HiDPI support is spotty
The log in screen as well as some other components of the OS do not scale properly under HIDPI. Some things in the log in screen will be displayed too small, such as the mouse pointer. It can also be difficult to get full scaling to work properly in the DE itself with things such as icons, text and window borders.
Con Too complex
Shows too many options at once often time, making it more complex than simplistic.
Con Difficult to use on virtual machines on version 5
KWin compositing is restricted to xrender on virtual machines which makes the default booting process difficult as 3D graphics needs to be turned off from the VM itself.
Con Difficult to turn off some transparency
Some of the transparency settings for Plasma can only be removed by changing away from the standard theme altogether. A bit disappointing as so many other things are configurable to the deepest detail and transparency in the wrong place can make reading menu entries for example difficult at times.
Con Held back by dedication to emulating Windows
One of KDE's pros is that it works similarly to 90's-era desktop environments such as Windows. However this holds it back from being able to present something that works intuitively for people who aren't familiar with how computers back in the 90's worked.