First time using Mental Ray ever since 2009! I've been sticking with Vray for the last 3 years, but after giving Mental ray a shot, purely out of curiosity which was in turn spawned from boredom, I realised I was wrong to neglect 3ds Max's native renderer.
I bet there's a ton of statistics and spec comparisons scattered throughout the web on this subject, but after a hands-on approach, I can see why. Both can deliver comparable results, but through different means, and I think I haven't yet understood each one's pros and cons, but I've understood that anyone seriously involved with rendering and shaders must have studied both before feeling competent enough.
Mental ray's depth regarding materials has left me shocked and awed, and I'm scavenging the internetz in search of good mr tutorials (material-related or not). So if anyone's aware of any good tuts, please drop me a line!