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.
Don't be satisfied with what you can do but rather strive to do the things you can't do!