This is useful for sparks (you can see some particles colliding in the image above), I also made a little point cloud shader that is also useful for sparks :D
Houdini - particle deformation blur and point cloud lighting from Sam Hancock on Vimeo.
The shader on the spheres and ground plane just loop through the particles and does diffuse, attenuation and shadows.
A single frame took about 6.5 minutes to render on my laptop ( Core 2 Duo 2.5GHz ) compared to about 1.5 hours for the equivalent PBR render.
hipnc is here.