For anyone interested in compositing using EXR files, Exr.IO is an invaluable free plugin for Photoshop.  It facilitates the process of importing EXR files with multiple passes.

Each rendered pass is applied to a separate layer that can then be adjusted in post.  A very clean and efficient way of working, since each pass is saved within a single file; helping avoid the convolution involved when rendering separate files for each render layer.

Andrey Lebrov also has an excellent breakdown, explanation, and tutorial on using the plugin and his related workflow here.

Below is an example of how some of these rendering passes are used to create the final image, as seen above.