And once you're used to it you'll never want to go back. All of my experiences with focus-follow-mouse on commercial OSes have been almost-but-not-quite right.
> And once you're used to it you'll never want to go back.
Eh. I've used focus-follows-mouse and tend instead to strongly prefer the OS X method of allowing scroll events to percolate to the window under the cursor but retaining keyboard focus on the window it's currently in. This allows me to scroll, say, a web browser on my right-hand monitor, while continuing to type in the left.
Hover-focus is great on tiling window managers, but I'm not sure I'd react well to it where windows overlap by design.
Also, don't forget that OS X lets you scroll whatever window your mouse is over, even if it is "blurred" (not in focus). This is the one feature that I miss most whenever I need to use Windows.