Yeah, Mental Ray is sweet, but too many people knock the Maya renderer without ever really "using" it. Besides the renderer in XSI, which is Mental Ray, the renderer in Maya beats the others hands down. Problem is the default settings are terrible, it's slow, and takes some time to use it well. If you have the cash, the best bet is Mental Ray AND Renderman :p
Red bellows of flame have blackened my stones
Convulsing my frame and cracking my bones
Hell's dragons of steel who roar in their chains
Crawl into my caves to suck out my veins.....
-The Mountain P.F.M.