Pros:
Reasonable price, costs only $50 dollars
Very easy on your computer, Sai has never crashed on my computers even with a ton of other applications running in the background
Great for lineart. The Pencil and maker tools can be easily modified to make super-clean lineart brushes, and the lineart layer tools can be used to make easy vector lineart.
Very simple to learn, it took me just a few days to get a good handle on the program
Cons:
Limited in function. Sai lacks text, word bubble, and paneling tools. Can't do fancy brushes like Photoshop, and doesn't have many layer modes.
Can only work in RGB, which basically means your colors will look different if you try to print them.
Hope this helps!