The following is a transcription of one of my independent projects while I was a student at SCAD. The original page is still available here. With RPS 17 and beyond, most of these features are standard, and I haven't continued development as I no longer have a RenderMan license at home. I hope this information is useful!
The Lambert model has been around for a couple of hundred years now and correctly describes how an object is lit based on the surface normal and light position (view position does not matter. Lambert's BRDF is just a constant). By observation, even a diffuse surface does not follow the Lambertian model. A good example is a full moon. The moon is lit very evenly and appears from Earth to have a smooth(ish) surface. However, the light does not fall off around the edges in a way consistent with Lambert's model.
From the previous explanation, we know that surfaces are generally made up of microscopic facets. It would be appropriate to assume that Lambert's model is correct - for these small microfacets. The distribution and visibility of these microfacets defines what the macrosurface looks like under observation. In simple terms, the surface does not darken as much around the edges, because if the surface is rough some portion of the microfacets will still be reflecting light at the viewer. In my shader, I have implemented Oren-Nayar diffuse shading , a popular model for describing rough diffuse surfaces. In the image above, I am using a roughness of about 0.5 - a value of 1.0 appears as a nearly flat disc (like the moon) but I felt it was too extreme for the moon example image. A roughness value of 0 indicates the surface is completely flat and thus is identical to the Lambert case.