Once a polygonal mesh is defined by marching pyramids, it has to be rendered using shading. Each vertex in the mesh needs a normal for smooth shading. Luckily, in the case of metaballs, these normals are just equal to the gradient of the energy field at each vertex. The gradient can be found using finite differences. The iso-surface can be rendered using either gouraud shading or environment mapping.
Environment mapping uses a texture to approximate the environment which is reflected in the object.
This is a simplified version of the usual environment mapping equations which convert the normal into spherical coordinates. I find that the visual difference between the two is negligable. The picture below is the picture I use for my environment map. It looks vaguely like the sun in the sky, and produces a nice metallic chrome effect. One reason marching pyramids is not used as often as marching cubes is that it can generate twice as many triangles as marching cubes does.