It recently got a big update, and now happily runs without X11. So it's better than it's ever been. If you've used Gimp on other platforms, I'd recommend it, but if you're just starting out I'd recommend something else.

Better (though paid) alternatives for OS X are Acorn and Pixelmator. Pixelmator is a more traditional Photoshop-like UI, while Acorn (in my opinion) is nicer, with a one-panel design that gets out of your way, while still having a host of powerful tools and scripting support (with a choice of languages: AppleScript, Python or JavaScript/JSTalk).

I recently moved from Windows to Mac and this was one of my top 5 problems - not having a decent image editor. I used Paint.NET while on Windows for the occasional "magic wand" work but on Mac every app was either too limited to use or . I've tried GIMP (pre-2.8) but its "multiple windows" and crooked X11 was a huge turn-off. With all that being said, the new GIMP (which is in a single window) was a huge surprise. Since I installed it, I haven't looked back at my old Windows desktop machine.