The collection of pre-fab Looks was essentially made obsolete with the introduction of Slim. Slim provides a quick and easy interface for generating custom RenderMan Shaders without programming. If you happen to have the old looks, they will generally work with the renderer.
Looks are really just sophisticated shaders . . . shaders with a little more "oomph." Looks provides many parameters for altering the appearance of an object, but they do not support shader hierarchies, like Slim.
Pixar Animation Studios