I've been using GIMP since at least middle school. I learned about color, layers, opacity, paths, etc all from GIMP. It has always been free, available, and it doesn't change too fast. I have used photoshop for advanced features (CMYK halftoning), but that never comes up for me often enough to subscribe.

Having learned on GIMP, its super easy to me. I have a hard time understanding what could be better just because I am so used to it.