I worked on TRON LEGACY at Digital Domain, creating Houdini fx for about 35 shots. The bulk of which were fog and other background elements. But the majority of my time was spent on three "de-res" shots, where killed characters broke down into thousands of little cubes that simmed and spilled across the ground.
This was some of the more taxing work I've done simply because the Houdini setup required multiple steps that provided lots of opportunities for things to go wrong. Pushing through 5 characters in 3 shots was a bear. For the few of us with these de-res shots, there were thousands of nodes to babysit.