If you have to ask...
But seriously, GIMP is a masochist's wet dream. You can't even access the menu without a gazillion mouse buttons. Linux weenies call it free, but I consider it more expensive than Photoshop 6 given that time is money. I once sent a smoke image to a GIMP user, he couldn't even see it. Seems X11 lacks something called ColorSync, whoops.
GIMP makes you appreciate the zillions of technologies Apple supports which we unknowingly use every day.
X11 apps in general make me appreciate the ability to copy+paste without the keyboard or a gazillion mouse buttons (I only have 4 for crying out loud).
hrm, what was the question? |-)