For a long time I've used Gimp, mostly because I run linux on my Desktop and although I can run Windows programs fine in Wine, they don't give me pressure support from my Wacom Graphire tablet.

Gimp suffers from the same problems as PS, it's really a photo editing application. But I recently bought a laptop. A very special laptop. It's a tablet PC. No not a sleek trim Ipad, but a complete laptop, 12" screen 80 gig harddrive 2 gig's of ram, Windows XP pro. What makes it special is it has a wacom tablet built in to the screen and the screen turns around and folds down. Full pressure support with a genuine wacom untethered pen, and the ability to run Windows art programs.
So what? Why is this better than a cintique other than the obvious you get a laptop at the same time? Because it was only $275 used. Yeah, in 2005 it was $1500. But now the
laptop portion of it is seven years behind the curve. It's all about 1/2 inch thick I-pads now. All the biznes folk don't want these anymore and have dumped them on the used and refurbished market. They won't last long there, so grab one while you can.
Now one big problem with this is when you fold over the screen it covers the keyboard. Yes you can draw without it, but those hotkeys and modifier keys are important. You may only use two or three of them for some art program but drawing is labor intensive and they're important. But I just plug in a USB keyboard and no worries. Haven't had much time until recently to use it for art though.
Since GIMP sucks hard on Windows (Windows doesn't do a lot of the window handling stuff X-windows does for you which makes Gimps interface seem really awkward, while it's awesome in Linux, plus it's slow on Windows). So I've been trying out other software. Since this system is only 1.4Ghz single and single processor, lighter weight apps are best. I've been using MangaStudio EX4 and Easy Painter Sai. Both let you spin the drawing area easily which is important because like a piece of paper and unlike a traditional tablet, your hand can cover your art work while you draw, and the tablets are NOT as easy to spin as a piece of paper.
Both handle sketching very well, come with neat brushes (ho-hum), and when it comes time to ink, they both provide free hand pens that are stabilized for awesomely professional looking results if you want to use that. MangaStudio also comes with all sorts of bizarre extras, but I don't really use those.
Anyhow they both have free trials so you should check them out, especially Sai painter. Just google it. And it's only $40 if you decide to buy it instead of pirate it forever (which is easy but I bought it). I also thought SketchBook Pro was pretty cool. I loved it's interface, (you really do NOT need a keyboard with it until it's time to type a file name) but the tool set left me little unimpressed.
D-D