I have used mentalray mainly for architectural design purposes, and I find it very good at that (max 8 ).
I tested max 9 and they improved the architectural materials templates (available in max
BUT i think we are loosing so many advanced aspetcs of this renderer due the lack of documentation, specially regarding lume shaders and custom materials.
I would like it to have better lighting setup and capabilities (maybe it already has it but I haven´t figured out). An example is light maps or light objects available with vray; why they arent present in mentalray?