One section has a rather odd name for the chapter- the "symbiotes" are actually organ transplants so that the recipent gains some of the power of the donor (be it ability scores, ex abilities or senses). The basis of the name is that the original researchers looked at symbiosis and discovered the 3 main forms and studied how organs could be mutalisitic (symbiotes that help each other). Strange, but it works sort of. 11 1/2 pages of text are devoted to it- half explaination and rules for surgery and the other half on individual organs and tissues.
The reason it is in WW has to do with the required substance for keeping the organ alive and acts as an analgesic (sp)- a drop of Haiel's blood (which is of course, very expensive).
Of course with something like this there is a ton of math- organ removal, implantation and chance of rejection. But it isn't difficult math and unless one of the PCs becomes a symbiote surgeon, it is rarely going to be needed.
Overall I find it much better than the Fleshwarper in Lords of Madness.