I just recently downloaded and installed GIMP. I'm finding it a bit clumsier than Photoshop. But that's probably because I've been using Photoshop for the last 20 years.
I like GIMP, despite its color depth-issues. And even those aren't much of a problem since I don't often work with RAW image files and have other software for that.
Another package you might like is MyPaint
. It's my go-to paint package for when I want to rough-out ideas.
For structured, vector graphics, there's Inkscape
. I use this for a lot of icon work and any time I need crisp lines without any jaggies.
I'd kill, though, for a modern equivalent of DeluxePaint III from my Amiga days.