I spent a couple of weekends working with Spiral Generator for the One Synth Challenge. After figuring out what this is capable of, I'm in love. And Free.
This synth is not for the faint of heart, but if you invest the time you'll find it a serious add to the toolkit. Screaming leads, Metal basses, and all manner of chaos.
When learning, I'd recommend running audio through a vectorscope so you can see what is happening with the spirals, and also you can see the effect of the main parameters. Morph and Density are the main ones, and Z-Depth and Z-Amount are great for dirtying up any sound.
Awesome and innovative! Brilliant, unique sounds, SO much fun to get my tweak on.
9/10, only because it's a bit unstable in FL Studio for me.