To add to the above:
Mental ray doesn't support PTex, you have to get Renderman or any other PRrenderman enabled renderengine
Mudbox and Mari have Ptex support and I personally don't like any of these 2, Mari is getting better, but it takes forever to get things done (at least for me) and you need pretty decent hardware.
I also find it a bit to technical, for texturing I still prefer Zbrush and PS, but it is very impresive with 8K uv patches , displacementpainting -you" re painting a map and not sculpting geometry.