The pencil tool in Alias' Sketchbook pro is the only real reason I use the program. It is by far the most natural sketching program. I think it has to do with the automatic tapering of the stroke. Even if you are using a mouse it sort of fakes pen pressure sensitivity, because it will taper off based on the speed of the input tool across the canvas, as well as the amount of pressure. I've never seen any other program do it, and it makes it a really good program for sketching. If you guys could figure out how they do it and incorporate it into your program I'd have no reason to use any other program.