In my 2d Illustration class we recently learned how to use the mesh tool.  In the beginning I was kinda iffy, but now since I have been using it more and more I've grown to love it.  You can make so many images come to life just by little detail into the picture that you are creating.  For instance with this picture of my self portrait  I used first the pen tool to trace the image and add color but then used the mesh tool in-order to create, highlights and shading to make the image for dimensional.  Now I am not a pro at this tool yet, though I would love to be.  There are somethings that me personally I have to learn.  Just recently today It clicked that i can add different colors at different points to make the image more appealing and smooth rather than having ruff edges.