PaintTool SAI is fast and easy, everything you need for drawing and painting is already there. Next to this you can create your own brushes and edit options like density, opacity, texture, shape and the amount of blending. Talking about blending: I found that this works a lot better in SAI than in a lot of other drawing applications. Next to that, you have a build-in stabilizer for smooth lines. For me, as an illustrator, SAI is absolotely very useful. You can also save in Photoshop file format, so you can continue editing in Photoshop if needed.