Here is a simple file that comps three simple (and ugly) objects using diffuse, reflect and shadow passes in Mantra. These passes are set up in the render output nodes and you will find one render output per object in the scene. The other objects are set up as mattes in the output node. By rendering all the passes in one go like this instead of using takes and different render outputs per layer, you save on render time and still retain control.
In the comp network you can see how the passes are pulled out of the .pic files using channel copy nodes then recomped using adds and multiply nodes. The shadow is interesting. You render out a shadow pass and diffuse with no-shadow pass then multiply them together. The result is then added to the scene. By altering the Foreground weight of the multiply, you can determine the strength of the shadow. A weight of 0 means the shadow is used completely.
Check out the file and if you have any questions about how it works then let me know.