Personally, I didn't care for GIMP that much. I admit that I didn't really study it, but my impression of it was that it would be quite hard to do any precision work in it. For example, I wanted to smooth part of a scanned page. However, I only wanted to smooth an irregular image in the center of the page. Trying to outline the image with the entire page visible on the screen wasn't very precise, but once you zoom in and start a freehand selection, there's no way to scroll the window to select a larger portion of the image than what's currently visible. Also, the selection and erasure methods seem pretty limited. It does have a good selection of filters and color operations though.
For actually editing images, I prefer Ultimate Paint, which is available in both shareware and freeware versions. The only thing missing from the freeware version are a bunch of the image filters. You can get it here;
http://www.ultimatepaint.com/
Think of it as Deluxe Paint for Windows.
Things I like: For all tools, left button draws in the foreground color, right button erases to the current background color. You can instantly cut out a brush and draw with it, no messing with COPY or PASTE menu commands. +/- on the numeric keypad change the zoom level, no need to click icons to do it. Screen can be scrolled or the zoom level changed without interrupting the current operation. You can change the 'handle' on a clipped brush between the center and all four corners by pressing the "5" key on the numeric keypad. You can add or subtract pixels from all four edges of the canvas if you find you need more room or want to get rid of unused space. Most all the features just seem to work like you'd expect them to.
Things I don't like: There's no transparent fill! Holding down the Shift key to constrain movement to the horizontal or vertical doesn't kick in right away, pretty much negating its usefulness. If you zoom into a window, switch to a different window with a different zoom level and then switch back, the first window will reset to the upper left corner of the image and you'll have to scroll back to where you were working. There's a transparency setting for brushes, but it's really only good for stamping down a single copy. As soon as you move a brush in drawing mode, it instantly becomes 100% opaque because it interacts with the color it just painted.
Despite the few negatives, it's my graphics program of choice. It may not replicate drawing on rice paper with colored chalk, but in my opinion, you can't beat it if you want pixel-level control over your image editing.