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Awesome always works as it should: it is very stable and reliable.
Pro Fully extensible with Lua
Pro Low latency
Awesome was the first window manager to be ported to use the asynchronous XCB library instead of XLib, making it much more responsive than most other window managers.
Pro Good default configuration
By default, you'll have a status bar (hidden in some modes), an application launcher, automatic fullscreen, manual fullscreen shortcut, etc.
Pro Tags instead of workspaces
With awesome, clients are organized with tags: one client can be on more than one tag, and multiple tags can be displayed at the same time.
Pro Highly configurable
Awesome is highly configurable, allowing the user to change anything they see fit in order to make the WM work for them and their workflow.
Pro Some mouse tiling support
You can rearrange and re-size [some] panels via the mouse.
Pro Xinerama support
Awesome has real multi-head support via XRandR/Xinerama, with per-screen desktops.
Pro Single window, multi-workspace support
The user can have each window visible on one, multiple, or no workspace. You can also temporarily include another workspace in the current one.
Pro Easy module for useless gaps
Lain module makes useless gaps easy.
Pro Any window can be full screen
Select the window. Hold ALT+SHIFT and press SPACE until the window takes up the entire screen.
Con Difficult to google for solutions to problems
Awesome is a very common word, making searches for solutions to problems using Google very time-consuming as a lot of chaff has to be sifted through.
Con Regular API breakage
Results in many of the scripts for awesome to be found online end up being outdated.
Con You should have some skills to configure it
Awesome, like most window managers, is targeted at advanced users. Though is has sane defaults and easy to read documentation, it is still a far jump from the more common graphical UIs found in computing.
Con Doesn't have tabbed containers
There is no option to have tabbed containers in awesome window manager.
Con Some programs don't cooperate well with tiling window managers
The user can usually work around this, but it can be quite annoying at the same time.
Con Re-size buggy
This is more a case of personal experience with awesome, that some Windows are not re-sized properly.
Issues can include: terminals not re-sized to the bottom of the screen to re-sizing back to 1 pixel size, awesome cannot move windows or re-size panes, and awesome doesn't re-size contained windows properly (observed with Hangouts Chrome extension, for example).
Con Configuration uses Lua (Programming Language)
It is time-consuming to make changes to configuration. Though Lua is a good language, a plain text file to configure things would seem to be a better approach.