mental ray 1.5 is not a new renderer, and it is not exclusive to maya.
Aliaswavefront is selling mentalray 3.0 (for maya 3.0 4.0 and 4.5)
but it's a mental image product, it exist for 3dsmax and it is bundled with Xsi.

it's a wonderfull renderer, it make your render photorealistic if you know how to use it but it cost a lot (6999$ can)
but it worth it!