I was pleased. I can best describe the effect as a ghostly colorful vortex. It's very pretty & trippy, and meshes well with other techno effects.
This instructable describes my first prototype. While still crude, it's effective enough that I've made three duplicates so far, and plan to deploy as many as I can make at the upcoming Critical Massive festival, which is the Seattle-area Burning Man community "regional burn". I'll likely also bring a variant of this to Burning Man itself.
My camera works pretty poorly in low light, so the in-action pictures and video barely do it justice. It's most fun when viewed from below; we've spent far too much time already lying on the basement floor gazing up at the pretty swirl.
OK, on to the instructions.
Step 1: A peek at the goal
The blade is attached to the motor using a collet, and the motor is secured to the ceiling using a hose clamp, as shown in the second picture.
There's a thousand ways to accomplish this task, and by no means am I claiming that this is the best way. It was, however, what I could quickly accomplish using what I happened to have lying around.