The reason why these shaders are the dog's bullock (what is a bullock anyway?) is because skin doesn't respond to light like plastic or metal does as stated by wdupre. Translucency alone isn't sophisticated enough to reproduce multiple levels of a surface. Poser (6) SSS is a fair compromise but is really nothing more than a trumped up BDSF shader with no "depth" per se.

Regarding mental ray's physically accurate SSS shader, I can't attest to how long it takes because I could never figure out how to plug in the right numbers because I'm really not trying to render glasses of milk or catsup.

fast skin SSS, while not physically accurate, produces phenomenal results IMHO that are fast (can be used without raytracing), and subtle yet remarkable. Currently using Renderman because mental ray's motion blur is too slow. Pixar's SSS implementation is much less intuitive than mental ray's.