I use both ArtRage 3 Studio Pro and SketchBook Pro.. and recently I bought Paint Tool SAI. Sometimes I use excellent free painting program MyPaint. These are my main drawing and painting applications.
I use MyPaint 0.7.2 also on my Nokia N900 (OC 900MHz) Internet Tablet. It is amazing how well it works.. It is my mini Cintiq+Painter. Great digital sketchbook.
N900 resistive screen is perfect match for painting applications. Screen is accurate and works just as well as my Wacom Cintiq 12wx.
These softwares gives me all that I need for painting and drawing digitally.. they have a own weakness and strengths. All these costs almost nothing, less than Painter.
I am Long time Painter user but I think that ArtRage offer much more realistic traditional tool emulations... and new Sticker Spray Brushes offer a whole new world of brushes. Stencils and rulers are very useful. Canvas textures are great and with them you can make endless amount of different type of textures. There is also Layer Blending Modes. ArtRages only weakness is it how slow it handles large canvases and I hope that this is corrected in future.
SketchBook Pro is so easy and fast to use. Pencil Tool has a great feeling.. Other great features are symmetry and ruler tools which are fast to use, transforming layers is so easy. Zooming and canvas rotations is fast and whole program is snappy. I like very much the new texture brushes which you can make and customize very fast. SketchBook is so good for speed concepts, but I think it is very useful for final stages or entire paintings or drawings. There is no Layers Blending Modes or canvas texturing.
I must mention Paint Tool SAI because it offer great Linequality (customized Stabilizer) and very very fast Color Blending engine. Also it is so light program it can handle very large canvases without any slow downs. It can rotate and zoom canvas fast. Brush engine is quite capable if you customize it. There is also Vector line tools and offer unique way to modify them. Paint Tool SAI is my only tool when I need absolute precision for my lines.
I still think that Photoshop is only tool for image-editing and best of them if you need quality.