* As far as I know, we can only run mental images' iray in NVidia's RealityServer. I hope Autodesk 3dsmax/maya/xsi supports iray soon.
mental images (a subsidiary company of NVidia) demonstrated iray renderer in GDC's keynote and SIGGRAPH ASIA 2009.
GDC NVidia iray demo (YouTube)
mental images iray page
mental ray or some other renderers can simulate indirect illumination in some extent, but the methods final gather or photon map are quite rough approximation. They have no guarantee to converge to the correct answer. Also there are many renderer parameters for performance optimisation. These parameters heavily depends on the scene. It is difficult to achieve high quality and faster rendering.
These methods are anyway time consuming, therefore, we sometimes put invisible light sources by hand, that simulates indirect lights. This iray is a brute force path tracer and there is no renderer parameter, but nowadays computer has not enough power to compute the light transport simulation in straightforward way. Then they use GPU. The demo shown in YouTube uses a GPU cluster. GDC demo shows interactive rendering with such a high quality. It is impressive. But you need 15 Tesla cards. (How much is it?) I heard Tokyo Institute of Technology has Tesla or Fermi cluster, does anybody know do they run iray on that? I am interested in that. SIGGRAPH ASIA 2009's demo showed five Quadro 5800 FX cards machine. (Still I can not afford it. But demo was nice I think.)
iray is a physically correct light transport simulator. At least it is based on more physics than mental ray. SIGGRAPH ASIA demo people explained that iray has only build-in shaders, actually only one build-in shader. Someone asked if he needs to use a custom shader, what iray can do. The answer was:
1. iray only support this world physics. Unfortunately iray doesn't have a plan to support other physics.
2. If you really want not non-physical stuffs, please use mental ray or ReallyServer's software renderers.
That's true, they said this is a physical light transport simulator and he asked how can he customize the physics. They also explain that the target customers are architect, car designer, industrial designer, and so on. So not oriented to the people in film production. Since artists rather cares good pictures than physically correctness. mental ray will continue and that's for the artists. After all, it seems
1. to the people who wants physical simulation: iray
2. to the people who wants customize the image: software renderer(e.g., mental ray) + MetaSL.
First time people saw iray, they were a bit confused. Why mental images will have two renderes? Because there are different customers and needs.