The monkeys have just got their first copy of Renderman, and have started to have a play with it.
It turns out that its blazin' fast in comparison to Mental ray, it will render hardware particles IN SOFTWARE! and for those 2 reasons alone its enough to burn all your copies of mental ray right now. Also its from those clever boys at Pixar, so you cant go wrong. Oh and its cheaper than mental ray.
What you waiting for! go forth and use Renderman!
More info when the monkeys have picked their jaws up off the ground.