I've used GIMP before (mostly because it comes prepackaged with Ubuntu) and while it's very nice for some things, I had trouble completing some tasks that are extremely simple in Photoshop (Even just trying to import two images, crop/splice them together, and save it was a challenge).
Also, for those who prefer to use vector graphics (In my opinion they are far simper to work with for most graphic design applications) but can't afford Adobe Illustrator, I recommend taking Inkscape for a spin.
http://inkscape.org/
And even if you've never used vector graphics before, give it a try, it's extremely easy once you first get the hang of it!