Interesting. I have never heard of GIMP, but I am not happy with my current pic editing software at all. I recently downloaded Picassa. It is free, too, and unfortunately worth every penny.
Gimp is more like Photoshop both in capability and complexity, but is open source and absolutely cost free.I use Picasa to get the pictures off of the camera and for quick, easy edits, but sometimes I want to do a little more than Picasa is capabale of or intended to do. There's room for both: Piacasa is easy to use, but Gimp has orders of magnitude more utility.