The young girl render is a wonderful render, no doubt about it and mental ray is indeed an outstanding render engine. This being said, not everyone using mental ray (even amongst professional) will be able to achieve such a result. With mental ray like with poser, in the end the determining factor is the artist sitting at the computer, her / his knowledge of the software, patience and artistic skill. 
One side note, a lot of what mental ray can do, poser can do it too (I discovered that recently thanks to some great tutorials by bagginsbill). The very big difference is, mental ray comes with very powerful shaders already assembled while in Poser you have a lot of the needed tools, but you have to assemble them. If you consider that, a Poser artist will need a lot more knowledge than a person using mental ray to get a photo realistic result than someone using mental ray but some people achieve outstanding results nonetheless :)