I have a Linux, OS X, and Windows machines on my desk (for doing cross-platform development of Firefox). XMonad is definitely my preferred WM for getting things done. OS X and Windows both have attempts at nice features, but they feel so frustrating to use in comparison.

Lion actually improved that a lot: Fullscreen apps integrated with Spaces gives a large part of what I use X<onad for. If they'd combined that with Windows 7's snapping of windows to half-screen, that would cover 75% of my XMonad usage. It's really a shame that XMonad is Linux-only. (Or that Firefox users expect Windows&OSX ports, depending on your perspective).