Here's a quick tip for Maya users:
You may have had trouble with maya viewports when using MentalRay shaders. They don't always display properly in your viewports... in particuclarly when using "combined texture" previews. I despise when I see a flat black shape, unable to generate a preview. Often renderLayers compound display problems, as shaders swtich from one type to another. Honestly I haven't yet figured out all the possible combinations of issues that break proper viewport display. Often I just need to place a hilite or some other utility light, and I just cannot get a proper preview of diffuse and specular.
So, I've taken to creating a "lightingPreview" renderLayer. This one I switch to, just to see overall light levels and specular positions/intensity. I usually give it a material override, unless individual colors are important. You can obviously apply any material/s you like to get the preview you like. RenderLayers can help you clear up your viewports and see what you need to, interactively. I've been finding this more and more useful as scenes get more complicated.