If intuitive is your problem, I wouldn't say that GIMP is the answer... I love the program, but like most open-source tools, it's more about functionality, and getting better access to tweakable qualities of the program, than it is about being easy to grasp.

That said, I wouldn't say it's less intuitive than PS, unless you already know PS. That's a very big "unless" - I didn't have any problem picking up GIMP because I never liked Photoshop very much, but my partner was a PS user for years before migrating, and he still (after about five years) comes up against things that frustrate him because they're handled differently, named something else, et cetera. Mind you, he also doesn't use the program even a fraction of how often I do, so I believe it's largely because he never got baseline comfortable with it in the first place.

Bottom line, GIMP can still do almost everything PS can, but it's often in different ways or under different names. If you're not already accustomed to PS, I would totally recommend it unless you design primarily for print (since GIMP still can't do CMYK colour - sadface). Otherwise, it will serve you well.

Like arsheesh, I would advise you to grab Inkscape as well, since vectors can be really helpful for some aspects of mapping, and both PS and GIMP are awful for vector work (understandably, since they're not designed for it).

In summary, why do I use GIMP?
  • I have no money.
  • I wasn't working when I chose which program to run with. That meant I didn't need industry standard (fortunately, with regard to the first reason :p).
  • I tried PS when I was a student, and I never liked it anyway :p
  • I like supporting open-source software for philosophical reasons
  • It's able to work with some pretty obscure file formats (like Kisekae Set system cels, which nobody has ever heard of) that are useful to me.
  • It has perfect colour-palette management, which is super-important when I work on sprites or KiSS cels. Back when I chose my program (like 1998) PS had really awful palette management, since it was angled at photo-manipulation rather than pixel graphics. It might be better now though, I haven't checked lately.