I've been using Xmonad for such a long time I can barely use a non-tiling WM properly now. Makes me look like a fool on almost anybody else's computer.

But perhaps the greatest thing about Xmonad is that it can lead you to Haskell and to functional programming in general, and I think this is one of the most useful and beautiful discoveries a developer can make. Your Javascript or your PHP will get better just by looking at some Haskell :)