When comparing KDE Plasma Desktop vs Common, the Slant community recommends KDE Plasma Desktop for most people. In the question“What are the best Linux desktop environments?” KDE Plasma Desktop is ranked 3rd while Common is ranked 14th. The most important reason people chose KDE Plasma Desktop is:
There are many customization options and possibilities to tweak the desktop, including widgets.
Ranked in these QuestionsQuestion Ranking
Pros

Pro Highly customizable
There are many customization options and possibilities to tweak the desktop, including widgets.

Pro Looks beautiful
The design of the three built-in desktop themes; Air, Breeze, and Oxygen, are very beautiful to some.

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 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, excellent removable storage integration, and an optional terminal panel while remaining fast and easy to use.

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 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 Enjoyable to use
Thanks to looking awesome, and being customizable and flexible.

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 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 Fast and efficient
Looks great! Dolphin file manager is without a doubt the best fully functional and easy to use and multitask with.

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 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 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 Activities (evolution of multiple desktops)
You can really separate your work environment from personal environment. Applications can be 'pinned' to a single Activity (spreadsheet for work, game for personal) or shown on all Activities (web browser, for instance).
Pro Great for developers
Provides its own IDE for C++, Qt, HTML and through workspaces allows better organisation of work.
Pro Integration with mobile devices
KDE Connect allows integration of the computer and mobile devices on the same Wi-Fi network.
Pro Low system resources consumption
Not as lightweight as XFCE, but pretty close (like +100MB in real use).
Pro Separate LTS version
KDE has an LTS version for people expecting stability.
Pro Not based on GTK
A lot of users don't really like GTK's style and way of doing things.
Pro Innovative
Has a lot of good features preinstalled (Android integration with KDEConnect, etc.), and comes with lots of improvements and new features with new releases.
Pro Touchscreen support
Works good with touch devices and allows customization of gestures for them.
Pro Easy-to-use software center
Apt, snap, and packagekit all show up in the built in software center. Updates show as an icon in the system tray from all sources.
Pro Familiar interface for transitioning Windows users
Pro Useful for low-end devices
As Linux moves into low-end territory with the likes of Raspberry PI, CDE's level of consumption seems extremely small. When it was first developed, 128MB or RAM was a lot.
Pro Mature
CDE was developed more than 20 years ago to work as a unified DE for all the various forms of commercial, proprietary Unix operating systems that dominated the market back then: AIX, TRU64, HP-UX, Solaris, IRIX etc...
Nowadays it's released as an open source Desktop Environment for Linux. It comes as a free tested, widely deployed and enterprise-level product even if it's recently re-release as FOSS for Linux.
Cons
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 Certain releases are frighteningly unstable
Con Too complex
Shows too many options at once often time, making it more complex than simplistic.

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 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.
Con Swollen look out of the box
In default theme elements are extremely large, which makes screen feel smaller than it really is. Some of the things are not easy to fix - even compact main menu still has awful huge paddings and header font size in calendar is ugly.
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 Shell-style ≠ widget-style
The Plasma-shell is unable the use the current Qt style for its interface thus making it hard to get a consistent user interface.
Con Add-on installation can be tricky.
Adding themes and widgets can be tricky.
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 Not for production
May be extremely buggy and there are unnecesary configurations which takes away time to do actual work.
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 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 CPU overheard
Con Not based on GTK
Con High RAM usage
Con Outdated UI
The graphical interface is very outdated and ugly. It's mostly for lower-end machines and for people who want to give their Linux machine a true UNIX feel.