Another way that I use quartz composer is in keynote to make a really cool object in a presentation or as a novel transition.
For a cool transition put a 'transition slide' between two normal standard slides with an automatic transition after .2-1s, depending on your quartz composition to make it look good. Then set the actual transition between all of the slides to dissolve, which usually works best, but feel free to play with it. This does take sometime to make it look smooth, but can be worth it.
For a quick demo of this make three slides with a black background. Set the middle slide to automatic transition with .5s delay and drap the 'particle system.qtz' or 'explosive.qtz' file from /Developer/Examples/Quartz Composer/Motion Graphic Compostions onto the middle slide. Set the first slide to transition on click with dissolve and start the presentation. This is rough, but gives you the general idea.
Takes a bit of extra work, but also can make really cool effects. If you take screenshots of begining and end slides you can incorporate them into the qtz composition.