I Did this piece back then when I decided to stop bootlegging software and was using GIMP because I couldn't afford Photoshop at the time.
I found out that GIMP does some stuff pretty well and offers a pretty good amount of control over key mapping some of the more advanced brush features like rotation and aspect ratio which I thought was pretty cool. Also, it can do image hose brushes and animated brushes which Photoshop can't do.
My time with GIMP was short lived though because shortly after I began beta testing TVPaint for linux which completely blew my mind and I now use it more than Photoshop for painting, drawing and animating. But I'll discuss TVPaint more in depth another day.