Could you possibly post an example of one of these animations? I have always been amazed by what can be done with mental ray in 3DS. Our team actually looked into buying the V-ray rendering engine but found it was way out of our budget, extended render times way out of our time frame, and it was unnecessary for our simple application.
As I have heard a few people say, you can achieve much more than you believe you can in MAX with mental ray. The flexibility of the mental ray materials and the speed of rendering (also throw in the cost in there) are an unbeatable combination.
This collection shows some renderings from multiple programs as well as multiple render engines, and I have to say that most of the models using mental ray look absolutely stunning. Most of that is because of fancy texturing or work with caustics (both of which I highly recommend getting acquainted with). There are plenty of tutorials on how to make your MR textures look amazing, you just have to keep in mind your rendering time (at least our team would).
If you have any questions I would be happy to answer them to the best of my ability (however small my ability is).