This is an astoundingly good piece of work. I've been playing with it and discovering cool things at every turn. It has one of the most powerful oscillator design sections I've ever seen in a VST - easily as complex and subtle as Absynth if not more. I keep clicking edit here and there and finding more editable paramaters - for example you can choose a formant type filter and then edit the formant.
The sound quality is very good, far better than I'd expect for a free project that is this far gone into complex propellerheadedness. The oscillators can be made to sound trashy when you create very complex waveforms but I didn't detect aliasing unless I pushed it really hard and played really high notes.
There are some very usable presets - in particular, some realistic reed instruments, pads and atmospheres, and miscellaneous odd noises. The presets could be more extensive and imaginative - there are worlds of sound design to explore here.
The VST works pretty well in FL Studio - just remember to minimize instead of closing the Zyn window. You can only load one instance of it but it's 16 part multitimbral, so you can direct 16 midi channels to it - in FL you give it a port number in the fruity wrapper, then create midi channels and route them to that port. I'm not using Cubase these days but you should be able to do much the same thing there. In Zyn you click on the button marked panel window to pull up the channel selector which also includes faders and knobs for the volume and pan of each channel.
The documentation is, like a lot of open source projects, not the best - it seems to be scattered and in the process of being rewritten. But it's not hard to pick up on what's happening in the program if you have some synthesis experience.
Interface is not the prettiest but it works well and I was able to find my way around - which is more than I can say for a lot of commercial VSTis.
I'll give the author top marks for support as he answers questions on a forum right here on KVR.
This is an outstanding instrument from a sound design perspective - I want to encourage the author to keep working on it.
I noticed that after clicking an instrument in any bank, it does not highlight well (using Linux) -- perhaps if I could use a different skin/theme this would be solved, but ZASF does not seem to pick up on my GTK, QT, or OpenBox themes. Is there any way to tweak the appearance?