Renderman is good, oh heck, great. Been a long time user, however other versions of the technology are pretty impressive. Such as 3Delight, Aqsis, Pixie, Air, several others are Renderman API and for interaction many of use Liquid. 3Delight and Pixie have been used in various features as well, not sure about some of the others. Many have unique features, some are just plain fast. All REYES render engines rock though.