Doesn't work, and when you think you uninstalled it, it stays stuck in your Library - Quicktime folder and will be used when you for example import an avi file into iMovie.
Which will result in that AVI file suddenly having this big ugly horrible "3iVX demo expired logo" over it (and I mean ALL over it!), and you wasted 2 hours importing a movie that before you installed and removed 3ivx would import just fine without the damned logo over it.
Stay away!