I worked in the past a bit in Mental Ray but that were when I still worked in 3D studio max. The biggist problem i have with external renderers is the time it takes. Normally you will look at the quality vs the time it takes to render and the if the client is paying for it or not. Even in blender (we use Yafray). I basically never use the external renderer. It is to time consuming and normally the client don't pay for this. So basically this will be my verdict. To compare quality to rendertime and then make your own desission. Hope this help you a bit.