I don't have the program but I have used the demo. Particle Illusion creates it's particles over your animation as it plays back as a video. It is a 2D program and doesn't react to your objects. Depth is faked by simply shrinking the particle emitter. You can create areas that will bounce your particles, as if it was actually hitting your objects, (think: shadow catcher odjects in C,) BUT, you have to have a good eye for doing that kind of work to get the same kind of results that you would have if Carrara's particles simply reacted within Carrara in the 1st place. All of that said, you can get VERY good results if you learn the program. This method is the same as most feature films, using a compositor like *smoke or some such. And the interface is very fast with real-time rendering for preveiwing/fine-tuning your effects.