I first tried XMonad about a year ago, and its since become an indispensable part of my standard Linux setup. It's an incredible productivity booster, especially on dual monitor setups, making it infinitely easier to rapidly switch between multiple windows. It makes me weep a little inside each time I have to boot back into Windows' single, non-tiling, keybinding devoid workspace (on a side note, I've used both DisplayFusion and UltraMon with some success for a more keyboard focused experience, so it isn't all bad. Still...)