i3 is a dynamic window manager that is targeted at advanced users and developers. It uses clean and documented code in order to allow for ease of configuration.
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Pro Comfortable usage (keyboard only)
The user keeps their hands in one spot (most of the time). One will find that the mouse is used less and less, making navigation quicker over time.
Pro Fully configurable (including tiling)
One of the biggest attractions of i3 is that it can be configured just about any way the user likes. Ranging from custom keyboard shortcuts to placement of opened apps, it is up to the user as to how they would like their window manager to behave.
Pro Easily readable plain-text configuration
i3 has plain-text configuration, meaning that no lua or haskell is needed. This makes it rather easy to recommend i3 to other people without worrying whether or not they have the knowledge to configure it as it can be read by anyone without prior knowledge.
Pro RandR support
RandR provides more information about your outputs and connected screens than Xinerama does. To be specific, the code which handled on-the-fly screen reconfiguration (meaning without restarting the X server) was a very messy heuristic approach and most of the time did not work correctly — that is just not possible with the limited information that Xinerama offers (just a list of screen resolutions and no identifiers for the screens or any additional information). Xinerama simply was not designed for dynamic configuration.
Pro Can be reloaded quickly and configured without a client restart
Just two hot keys: Shift+Super+C to reload the config and Shift+Super+R to restart (which takes less than one second). Restarts pick up new versions of i3 or the updated config file, so you can upgrade to a newer version or quickly see the changes to i3 without quitting your X session.
Pro Easily switch to and manage floating windows
i3 can allow for the user to manage floating windows. Floating mode can be toggled by pressing $mod+Shift+Space. This way the user can take advantage of tiling as well as floating windows, all in the same session.
Pro Fast, especially on weaker hardware
Tiling means there are no fancy compositing or window effects to take up system resources.
Pro Can tab windows
i3 permits tabbing through windows by turning on tab mode with $mod+w.
Pro No window gap
Space is not wasted by separators, therefore making the most of the confined space on smaller monitors.
Pro Can stack
i3 allows for stacking of windows in its environment.
Pro Sane development process
i3 uses test driven development with an extensive test suite to prevent bugs from ever happening again. All external contributions require a thorough code review to guarantee a certain level of quality.
Pro Terminal bell can be used to notify of completed actions
Terminal-bell gets passed through and marks the workspace visibly.
Con Poor floating window support
Sometimes this is necessary, even when the Dev rejects feature requests. Firefox child windows (option dialog) is an example.
Con No window gap option
You can do a lot with i3 but the official version lacks the option to have gaps between windows. However, there's a fork called i3-gaps that is almost the same as i3 except the fact that it lets you have gaps between windows.
Con Missing "include" possibility in config
While pretty good and easy to use for common tasks, the configuration language is missing "include" directive common in other languages. You can use a workaround - a shell script to config parts on demand. It would be best if this were built-in however.
This can, however, be fixed by using a shell script to generate config file parts on demand.
Con Documentation is online
Can't access it offline unless you download the page.
Con Manual tiling
The layout isn't automatic. The user must move panels manually and may indeed end up spending time on that rather than on working with the application.
Con The plain-text configuration may not be suitable for beginners
i3 is configured through a plaintext configuration file. While it's very powerful and easy to learn, it may not be entirely user-friendly for those who have never edited a text configuration.
Con It has some issues with transparency
Using transparent windows can cause them to crash.