I haven't used XMonad but I've been a heavy ion3 user for many years. Last I checked none of the available bolt-ons (including Sizeup which the author proposes

I haven't used XMonad but I've been a heavy ion3 user for many years. Last I checked none of the available bolt-ons (including Sizeup which the author proposes) would come anywhere near a true tiling WM. The only candidate that would even try to automatically tile windows was TylerWM - but that was buggy as hell. All the others only act on keyboard-shortcuts, which largely defeats the purpose. I'd happily pay $200+ for ion3 as a native OSX WM. The OSX window manager is just absurdly terrible.