Ranked in these QuestionsQuestion Ranking
Pro Highly configurable
KWin has a very configurable environment. Just about any option that you would like to adjust will be available in the settings.
Pro Integrated compositing manager
KWin has built-in compositing with options on how it performs that can be changed by the user in the settings.
Pro Beautiful interface
KWin has an assortment of attractive desktop effects, creating a rather beautiful interface.
Pro Offers desktop workflow
In an age where everyone seems to be moving to touch interfaces, KDE remains one of the last DEs that still caters for desktop users.
Pro Window effects
KWin offers an assortment of window effects, such as wobbly windows and window shadows/glow.
Pro Effortless VSync
VSyncing with NVIDIA blobs can be tricky (the dreaded tearing) but with KWin, video and OpenGL games display flawlessly.
Pro Can be configured to be similar enough to Windows so it makes life easier for fresh converts
While kwin is far more powerful, it can be configured to be similar enough to Windows - for people who just changed operating systems and don't really want to learn something new, it works great
Pro Readable code
Much better than any GObject based mess
Con Some effects are slow and jerky
Some of KWins effects (such as present windows) can be a bit slow or jerky, resulting in uneven fluidity. This is no longer true on modern versions.
Con Dependent on some KDE libraries
This makes stand-alone KWin somewhat inconvenient to set up, as opposed to openbox and awesome, to name but a few.