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 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 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 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 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 Simple by default, powerful when needed
For the new Linux users (coming from windows), they'll find everything easy and simple to use!
But for old and experienced linux users, they can customize the interface as they want!
That's why Plasma KDE, Simple by default, powerful when needed!
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 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 Powerful window management with configurable rules
Hiding a particular window's decorations, automatically or with a keyboard shortcut, showing a window on selected or all desktops, and lots of other rule-based features are easily configurable directly from the window manager's UI without scripting.
Con Breeze theme is ugly
Breeze theme gives some wow effect at first with all it's transparencies etc. But when you get used, you start to notice lackluster icons and oversimplified look of ui elements - there is just nothing to look at and it looks lazy
You can install other themes, like numix and aero glass. You are not locked to it.
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 Kirigami is buggy
The newer QML based interface is too much tied to the breeze style. So if you use another style(like Qt's default Fusion) you will have a mixed desktop interface of fusion and breeze. It also fails at certain points to set the correct color in a widget so if you use a dark theme you will often face dark text on dark background issues.
Con Compiz integration destroyed
The KDE developers seem to pride themselves on destroying previously working features with each release. Plasma 5 is terrible because Activities and Virtual Desktops are completely separate now. VD's in Plasma 5 can't even have separate wallpapers let alone widgets. Activities don't integrate with Compiz and KDE well at all (no cube, different keyboard commands). Using Activities and Virtual Desktops together gets confusing real fast. Combined with the fact the developers have no interest in fixing this at all KDE users are just running away to new desktops that are borrowing old perfectly good KDE code.
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.
Con No native package manager frontend
KDE discontinued all of its native package manager frontends like: Kynaptic, Muon or Kpackage and fully relies on PackageKit, however PackageKit was mainly made with the RPM(Red Hat Package Manager) in mind and does not support all the features of more advanced package managers like dpkg or emerge/portage.
Con No easy way to backup & restore all settings
Most crashes cause loss of settings - panels vanish, all favorites and launchers missing, icons lost, wallpaper back to default, theme changes, activities present but not active, etc.
The configuration is held in a multitude of places, the changes are applied automatically even after a crash, all without a tool or clear way to recover.
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 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.