Well, I've checked out a lot of Mental Ray renders including visiting www.mentalimages.com since mental ray is distributed by mental images' OEM partners. I checked out their gallery (with a completely open mind) and for me, I still believe Maxwell Render is MUCH better at rendering photo-realistic scenes.

It's more time consuming to render with Maxwell, but maybe it's like food. A microwave is really quick but it sure can't stand up to the taste of slow cooked meal.