Ahhh, the G-Force plugin ... I remember it well from my WinAmp days. Absolutely the best abstract visualizer for WinAmp, I'm glad it's come to iTunes. Granted, I haven't had any music player just sitting doing visualizations for about half a decade, but I might just load this up and let it rip for old times' sake...
Apple's own visualizer is of course a modification of G-Force. I like Apple's album art display, simpler controls, and speed, but the "real" G-Force has more features otherwise. Resolution control, more effects, etc.
The "real" G-Force has been out for iTunes forever--but do get it, it's fun! If I could only have one visualizer, this would be it.