hi najafi,

mental ray is a killer renderer, as is lightwave's; the lightwave renderer is the best "out-of-the-box" one IMO. But there's some shortcomings still; there are no physically corect lens shaders (as in mental ray), and the radiosity algorythm (what does it take to make an american vice president dance?) seems to be not overly well suited for architectural interior shots. But on the other hand the HDRI support just rocks.

I have no renderman experience at all, unless you count BMRT in there; I played a bit with that one while still on blender back then.

As far as I know, the new Renderman (due this fall) will have raytracing and radiosity as well, but I haven't seen any renders yet. Mental ray is said to be quite slow, while Renderman is optimized for speed to a higher degree.

Never heard of JIG, what's that?