On Linux, I use mtPaint whenever I can, because it is quick and intuitive. So when I need to use GIMP, it is for a relatively complex task. I keep a GIMP-instructions text file to record every step of these operations so I won't have to research it again the next time.

My colleagues on Windows have been tempted to try GIMP (for casual use), but none I know have stuck with it. They revert to either PhotoFiltre or Serif DrawPlus.