mayb im just confused but:

FREE PUBLIC *BETA*
mental ray for Maya 1.5™

Download mental ray for Maya 1.5 and get your hands on one of the world's most powerful integrated renderers for Maya at no charge. Use the opportunity to use this FREE fully functional *BETA* version of mental ray for Maya 1.5 and evaluate it for your needs while checking out some of its hot new features

got that from the AW site where u download it from

i admit i didnt read properly the bit about it being free, but from what ive read its a beta

ok, why i dont like mental ray, i used the built in mental ray renderer when i used XSI, it was hella good! but then i saw someone use it with maya, it was hella not good thats it basically, the guy seemed to no what he was doing, the result just didnt look so flash, seems hard to use in maya as well, XSIs one is much easier to use