maya software renderer by comparison to renderman is bad because it is much slower than the rest and can introduce rendering artefacts.

Renderman is the renderer of choice for most big companies because it is very open and can be slotted into a production pipeline with relative ease. It also offers better quality and scalability (try doing a 2k frame in renderman and then try it in maya software renderer!) Only thing is renderman is expensive whilst the maya software renderer is free.... with maya.

Alan