Recs.
Updated
SpecsUpdate
Pros
Pro Very high work efficiency
This tile window manager can improve work efficiency. You can use many keyboard shortcuts to complete your workflow.
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 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.
Cons
Con Bad defaults for focus/moving
More of a nit-pick than an actual con, but still:
Devs love vim, and it's movement keys (hjkl) are an institution. So much so that emacs devotees use evil mode just to move around with the same ease. I would've liked i3 to default to the same keys for focus/moving to windows. Instead it defaults to the same keys, shifted to the right by 1 key (jkl;).
j is down, not left! Honestly...