My personal experience with this software is that it is very hard to learn. Now my visual prowess is very limited (I am a partially colour-blind person) and this makes it harder for me to work with graphical software in any case. However there are other softwares that are much easier to learn.
And in that sense I am talking then about e.g. InkScape (the vector-drawing-program) and perhaps also about Firefox and LibreOffice, but mostly because they turn out so well on a Linux desktop.
The good part is that these softwares turn out much better on a Linux desktop (e.g. KDE/Kubuntu). The bad part is that they are still the same programs :p.
But from a Windows point of view, criticizing these programs is really a misnomer. They are not designed for Windows in the slightest. For instance, the multi window mode of GIMP comes out much better when you can easily create a second desktop instance (workspace) and just move the windows there. In KDE, selecting the GIMP window then from the task bar will instantly switch the deskop to the second workspace. In this way you are grouping your windows, and since Linux has always provided this feature, it is common to assume that the program was designed around that.
The KDE desktop also allows for various kinds of advanced Window operations, such as "keep on top" or making one window a tab of another window. Even from a different program.
I cannot say so much about the graphical capabilities, as I am approaching this from the viewpoint of a novice user. As a software, and from the viewpoint of a developer, it is mostly a tool that can be automated given enough programming skills. A detailed knowledge of the various tools (the raw, primary toolset) then makes it possible to create your own "advanced" or "compositing" tools, that you can obviously also create from scratch as a user, and just use via the visual interface. This explains perhaps my interest, because for me the stage where I am capable of doing this, might enable me to do something more useful with it.
But personally I must emphasize that working with GIMP is a real pain thus far and the biggest obstacle in my creative work from the viewpoint of not having to deal with other more pressing life issues. Which come down to "HELP ME, I'M DYING" and in that sense I can be excused perhaps for not writing the most excellent reviews here. GIMP is the biggest stumbling block and I have often wondered whether not to "bail out" and just start using the commercial tools and dash out the $10 or more per month to get the complete Adobe toolset, or kit. Which would obviously create instant bliss from a certain point of view, other than the fact that they would require (once more, for me) a Windows desktop solution. And also the fact that you'd become dependent on a dying brand (you cannot buy the software anymore, which means your investment becomes dependent on your ability to forever fork out €12 or €60 (for the complete set) just to be able to keep using it, but that's a different story) and there has never been a Linux version of any Adobe program I believe. Which then means you are locked to either Microsoft or Apple. It is said that "all the other important applications, like Maya, Modo, Nuke, and Mari – they’re all on Linux as well". There is a new competitor to GIMP and Photoshop though: it is called Krita and it is built for KDE, which means, essentially, Kubuntu.
GIMP is still of interest to me because I want to learn how to code for it and I want to learn basic graphical operations in mathematical terms, in other words, to use my programming skills in tackling a problem that is so much harder for me due to my physical limits. GIMP is a vehicle for me for learning Python and GTK. But as a graphical tool, it really leaves me in the dark. Or rather, perhaps, in the dust. I've found many times that I wanted to do simple things, or things that would be possible e.g. through Photoshop or other "smartphone-like" apps for image enhancement, the way the girls all do, and it obstructed me in making progress with it (or with them ;-)). But also for professional things such as writing papers and enhancing them with quality images, I've found myself potently lacking. It is an ongoing experiment for me.
Shareware graphics applicaiton
While GIMP might stand up to Photoshop CS4, perhaps, the content aware features in CS5 and higher blow anything GIMP can offer out of the water. Add to that the new perspective/distort tools in CS5, the content aware cloning, and the puppet posing; and GIMP begins to look like a weak competitor.
And you can rent a Photoshop license now. If you really can't spend the $10 dollars to do your work in a month GIMP is probably the program for you.