I use paintbrush too, but it annoys me alot. If you try drawing a line it always shifts one pixel in a random direction. And I always paint per pixel so it's very annoying to use...
Still searching for another better program...
If you have access to the Mac App Store, you can download Sketchbook Express for free. The only thing I haven't figured out how to do (or whether it can be done) is adding text to a drawing; Sketchbook Express can do everything else MSPaint can.