Even I support Renderman more than M.R., But I think M.R. is a good renderer.
I think if you want to release MR or RENDERMAN power.
You cannot only use U.I. to operate that. If you know how to use programming to operate MR and Renderman. Then you can realease the power.
I think renderman is more flexible than mental ray. U can change many attributes by yourself. However, I don't think MentalRay can let users change it.
Although I'm using renderman in company, I don't know programming. So that is renderman control me, not I control it. I'm wasting renderman...
M.R. you can write your own shaders, the program is very look like Renderman and Houdini.
So if you understand how to write renderman shader. You can very easy to catch up M.R. and Houdini shader.
So, think about to lean how to write shader in renderman. And Internet has a lot of information about this