Is SAI easy to use? It looks like doing line art would be a load easier in that than PS but how does it fare when working from an image loaded into it? Does it use layers?
Yes, it uses layers, and the glorious thing about it is that it has a white background even when everything is transparent. So you can work on just the lines without having some annoying checkerboard behind it or something.
The thing that makes it awesome for lines is that it has a stabalizer, which makes the strokes very smooth with no jittering or anything.