Ranked in these QuestionsQuestion Ranking
Pro Extremely lightweight
Openbox is a lightweight window manager that uses a little over 100MB RAM upon boot.
Openbox is used as a default window manager in few desktop environments so it offers more testing, more bugfixes, and more stable behavior.
Pro Highly configurable with a moderate learning curve
Configuring Openbox is very easy to carry out, simply by editing its few config files. There are programs that allow for an even easier means of editing these files by doing so in a UI.
Pro Defaults easy to quickstart
Openbox's set of defaults are easily editable to the user's liking.
Openbox does away with many traditional desktop elements, like menu buttons, bars, etc. and places everything in the (insanely customizable) context menu. If any of the missing pieces are desired, they can be added through others apps (e.g. tint2 for taskbar).
Con Default configuration is lacking basic features
When booting Openbox for the first time, the user won't even know they have actually booted into a window manager as there will be nothing of note on the screen other than a right mouse action.