I've used Ubuntu for a few years now, and I adore Gnome's simplicity. For someone who mostly lives in Emacs and Chrome, its a great Desktop: its simple, it (mostly) works, and it stays out of the way! They must do some incredible usability testing, since it usually doesn't frustrate me like Windows does. If it didn't have such a single-minded fixation on warning you about Non-Free Components (gasp!) it would be almost unbeatable.
xmonad is great and I used to miss it on OS X, but I've grown to appreciate SizeUp and Divvy for OS X. You get tiling functionality when you want it, and only when you want it. It's slightly more involved than xmonad but only slightly. And it's unobtrusive. They're certainly limited compared to xmonad but they get the job done for me. I only have a couple of configurations I use very often.
I looked into SizeUp, but it seemed to follow a different model than xmonad/awesome: it only tiled (and then, only in halves or quarters). What I like about xmonad/awesome is that there's a master window and all other windows are automatically managed for me on the other side of the screen; if I want one of my windows bigger, I just rotate it into the master slot. I really don't have to manage my windows, which I love.
Another one to try is Optimal Layout. It gives you tiling, hotkeys for resizing and moving windows, and a better Cmd-Tab (though you have to Alt-Tab instead, unfortunately), and the latest release supports rotating. I love it.
