VRay and Arnold are awesome. I came from a MR / Maya and MR / Max background and learned all I could absorb. Read books, learned some vector math, and sat under some pretty smart guys. I really liked Mental Ray because that was all I knew. Then I met VRay and realized they did so much right that Mental Ray got wrong, and Arnold for it's sheer simplicity.
After a year working with VRay, I had to jump back onto Mental Ray due to my companies' pipeline. Ugh... I understand that it's not Mental Ray as much as it is the implementation in Maya. After spending a bit of time with MR, there are definitely some cool new things that even VRay doesn't have implemented. I've been learning a bit about all the hidden goodies of MR. Just like the 3x3 animal style at In-N-Out, some of the best things aren't visible, requiring some scripts and string options. The easy way out? Download these and learn the newer features, such as Unified and the new IBL features.
These guys know their stuff and are open to help whenever you have a Mental Ray question.