For those who don't know what it is and want to try it out: Here's the SourgeForge page: http://zynaddsubfx.sourceforge.net/

For those who have used it: How do you use it and how would you rate it?

I used it quite a lot before I bought some proper software.

Personal opinion: It's hideous to look at, confusing to use, has poor and noisy filters, and does not produce a clean sound.

But try it anyways. You may like it, and since it's free, you only lose some time.