I have never seen human skin in Carrara that looks truly realistic, unless it was based on photo textures. Honestly, I dont' think realistic skin shading was a priority among Carrara users until us Poser users started migrating to it for rendering purposes. I think it's possible to get good skin in Carrara, I just don't think it's been something people have worked on achieving until recently. Part of this is probably due to the fact that Carrara didn't have fresnel effects or SSS before version 5, without a plugin that is. It's my understanding that both of these effects are needed for respectable human skin shaders.