I'm usually very forgiving of apps, specially open source apps. I myself am a promoter of the open source idea, and I have some software developing experience. I know that people working on open source software are actually, contributing their time and resources to a greater cause. I do appreciate their effort. But honesty, there is no other way for me to express this, but "Gimp really sucks!" It specially sucks on Mac. everything about this app is clunky and awkward, specially the user interface. It's slow, confusing, and none native. The platform independence is a myth about this app, which is quite understandable, considering the limited number of resources and personal available to develop and maintain such a piece of mess.
Gimp is one of many open source software that has sacrificed dedication to achieve generalization. It seems Gimp developers insist on tying Gimp strictly to X11 and GTK, in order to accomplish the goal of platform independence. This is a trade off, giving away user friendliness, dedication, and performance, in order to reach a greater audience, ouch!
Gimp is too complicated and over complicated. I remember a famous quote from Einstein-"If you can't explain it simply , you don't understand it well enough." Simplicity is a novelty in an app. The translation of that in computer world would be: if a user using your app can't accomplish complicated task through simple straight forward steps, then your app is not a user friendly app, rather it's a complicated machine friendly app.
Gimp is a nice idea that hasn't been thought-out good enough. It maybe a nice app on linux, but it's a crap that should be avoided on other platforms. I've been using it for over 3 years now, although lately I've been only using it occasionally as a reference, and still it feels as awkward and awful as it was the first time I used it. I don't think I will use Gimp ever again for professional image editing or painting.
Trust me, if you are one of the 95% non Linux users on the planet, don't waste your time on this big Hype, save yourself the headache, and try something else. If you are a Windows user, I'd recommend Pain.net, if you are a Mac user and want to spare a few bucks to get a quality app, I'd recommend Pixelmator. Although they appear to be abandoned projects but it might be useful to check out Pinta, Seashore and MyPaint too.