Recs.
Updated
SpecsUpdate
Pros
Pro Good documentation
Every feature is thoroughly documented (including examples), and documentation is kept up-to-date. For questions that are not answered by the i3 user guide, for example because they concern tools outside of i3, there is the community question & answer site.
Pro Can be configured without a client restart and reloaded quickly
Just two hot keys: Shift+Super+c to reload the config and Shift+Super+r to restart which take less than one second. Restarts pick up new versions of i3, so you can upgrade to a newer version without quitting your X session.
Pro VIM Style key bindings
You can configure i3 so that your keys for moving windows is similar to vim, for example, M-j to move the window down.
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 and most of the time did not work correctly — that is just not possible with the little information Xinerama offers (just a list of screen resolutions, no identifiers for the screens or any additional information). Xinerama simply was not designed for dynamic configuration.
Pro Comfortable usage (keyboard only)
Once you master the shortcuts, you can quickly navigate without having to move your hand off the keyboard.
Cons
Con Missing "include" possibility in config
While pretty good and easy to use for common tasks, the configuration language is missing the 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.
Con Manual tiling
The layout isn't automatic. You have to move panels manually and you may end up spending time on that when you should spend it on working with the application.
Con No window gap
You can do a lot with i3 but it misses the option to have gaps between windows out of the box. You can achieve it through third-party tools though.
Con No shortcut to switch between two recently used applications/windows
You can easily switch between two workspaces but not two windows (which are not adjacent to each other). The functionality simply isn't there and the dev refuses to include it as a part of i3 core. This can get annoying when you have multiple windows in the same workspace. There is a manual workaround though.
Con Doesn't handle transparency
Let’s face it: Using transparency (or ARGB background with transparent parts) on any window decorated by i3 is known to break. If you have translucent windows (like many terminals), or transparent area (like Hangouts Chrome App), you'll have some horrible artifacts. The workaround using new_window pixel
doesn't always work well.
Recommendations
Comments
Flagged Pros + Cons
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 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 Excellent documentation
Every feature is thoroughly documented (including examples), and documentation is kept up-to-date. For questions that are not answered by the i3 user guide, because they concern tools outside of i3 for example, there is the community question & answer site.
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 Configuration allow multi-monitor support
User can assign specific workspaces to specific displays as well as apps to workspaces. This makes possible opening set of most used apps with 1 shortcut always on the same screens.
Pro Windows can be turned into Tabs
i3 permits tabbing through windows by turning on Tab mode with $mod+w.
This shortcut can be changed in config file.
Pro Configuration allows multi-monitor support
User can assign specific workspaces to specific displays as well as apps to workspaces. This makes possible opening set of most used apps with 1 shortcut always on the same screens.
Pro Application binding
i3 allows to bind an application to be opened by default on a specific workspace
Con It has some issues with transparency
Using transparent windows can cause them to crash.
Out of Date Pros + Cons
Con Poor floating window support
Sometimes this is necessary, even when the Dev rejects feature requests. Firefox child windows (option dialog) is an example.