"GIMP is loaded with the up-to-date tools many demanding
professionals need: Bezier path editing with brush stroking, tablet
support, Heal Tool, alpha channels, multiple-undo History,
area-averaged eyedropper, PSD file compatibility, and a wackload of
other high-end tools that are impressive in their attention to
detail.
"This isn't my first time with GIMP--in my series of image
editing tutorials, I used version 2.2 alongside Photoshop in an
effort to keep the information as application-agnostic as possible.
Version 2.6 has added a lot since then, so you can rest assured
that it's a lot of program for the zero buck. I reviewed the whole
application based on how it's suited to certain common or
professional tasks that potential users might perform--testing it
over a span of a few weeks on OS X and Ubuntu 8.10. So how'd GIMP
do in the hands of a professional graphics guy? Read on."