I’m a GIMP user myself. Earlier versions tended to be crash-prone with any scripts, but the latest is very stable and has a very good 64-bit version. Make sure you get the GIMP animation package for doing gifs (if you want to do gifs) and the free GMIC and Resynthesize plugins because they are awesome.
GIMP can use Photoshop’s brushes, of which there are many inexpensive (Obsidian Dawn is my favorite) or free (all over the internet, but check licenses).
Obsidian Dawn has a bazillion brushes…and lots of good free ones. Some are even packaged for use in GIMP, but all her brushes I’ve tried (either the PS or GIMP packages) have worked well. And with the new brush controls in 2.8, most of them work even better…my favorite new control is full rotation on the selected brush. (yeah, PS has had rotation for a while but GIMP didn’t or had rather clunky work arounds). http://www.obsidiandawn.com/
Ron’s Brushes, offered here in the store work very well in GIMP. Ron has a free sample pack at Brusheezy…
The GIMP Registry has lots of addon scripts, filters, plugins http://registry.gimp.org/
Pretty much, you can do almost everything with it you can do with any other full featured image editing software.