I don't know how Fast Scatter works but "normal" SSS sort of assumes your mesh has volume, while Fast Scatter appears to treat a mesh as a surface.

In Renderman, SSS is calculated by what is best described as a "multi-level semi-transparent 3D depth map". Generating the brick map takes time depending on the density of your mesh, but can be cached and reused which is a huge plus. IMHO, the look of Renderman's SSS is not as impressive as mental ray's.  

In mental ray, non-physical SSS is raytraced and calculated based on "distance below the surface", with deeper distances having darker colors, lesser distances being very light. Strangely the docs say to color your top layer RGB 226, 255, 255 (a VERY light shade of blue) and let the middle and bottom layers provide the oranges and reds of fat, blood and muscle. It is very convincing looking and mildly fast for raytracing. Physically correct SSS is a dark art that seems to be poorly documented, takes forever to tune and render.