I have had a MacGimp CD for over 2 months now and have used it to touch up a few photos. First, it is an easy install. It puts XFree86 4.0.3, XDarwin, and GIMP on your computer. XDarwin is an application that allows for OS X and X86 to run concurrently. I have not seen a complete version of GIMP running native in OS X. Place the XDarwin icon in the dock and click on it. It starts up X86 which takes over the screen. Once the three windows for X86 appear all you do is type "gimp" in one of them and the application boots. I'm not heavy into photo/graphic manipulation but for $25 it suits my needs and I, too, am pissed at Adobe for dragging their feet and ignoring the fact that if it wasn't for Apple they probably wouldn't even be around. I digress, once in the X86 environment all you do to get back to OS X is press the Option/Command + A keys and, poof, your back in OS X. Getting back to X86 is simple as punching the big X icon in the dock.
GIMP may not be as intuitive to use as Photoshop nor have all the features of Photoshop (yet) but it is free. You can spend your time downloading and unpacking tar balls for X86 and GIMP but the $25 CD is well worth not having to put up with that hassle.