I've been using the demo since it came out and bought it a week ago. Therefore, my review should probably be updated after more use........Oh, and I don't give 10s unless something has no room for improvement :-)
-GUI Laid out in a clear way that I love. Everything is easily accessible and in a logical place. A small point, but I prefer its tabbed form elements to drop downs - I'm zipping around the synth making tweaks and enjoying it......for the most part :-) If I have one complaint its that the harmonics editor is too small. I have a good 19" CRT running at 1280*1024 and find it very difficult to set harmonic levels. A small nitpick, but when you have a synth as good as Sytrus then I think you should aim for perfection.
-Sound Excellent is all I should say, as sound is so subjective. I love FM synths and Sytrus goes a step further. For example, filter implementation is very good. For each of the three filters you can select how much of the signal is routed to low, band, and hi-pass, and you can also route it to the next filter in line. If you're interested then go download the demo. If you want adjectives that mean nothing......then it is 'glassy, bubbly, fat, chunky, sugary, opaque, semi-opaque, transparent, transparently opaque, virus-like' :-)
-Features Just some of the important features... *Unison mode for big sounds. *Spline-based envelopes for almost every aspect of an operator. The same goes for the filters. There is a lot of room for creativity with this synth. *Effects are good, though there aren't as many as some of the other FM synths have - just chorus and delay (3 units). *Oversampling up to 64x
-Docs Seem pretty good. I haven't had much of a need for them to be honest though.
-Presets Ships with 220 varied presets. There are quite a lot of usable ones, and then the show-off ones.
-Support Haven't had it long enough to answer this properly. Prior to buying Sytrus I fired off a few questions at Gol and he answered everything quickly and, more importantly, honestly. If I ask a dev if they plan on implementing a feature then I'd rather be given a blunt 'no' than a 'maybe when we get time'. That way I know what I'm buying, not what I might be buying. Also, IL has a lively forum where you can go for answers.
VFM This is my first Image Line purchase, so I'm not buying it because of some supposed 'blind loyalty' to Fruity products that some people hint at. Also, I bought FL Studio along with Sytrus, getting them both for $179. I love the sound of Sytrus. I've used demos of other FM synths for some time (I'm a slow buyer) and I believe that Sytrus has more features. The others do have some things in them that I'd like to see in Sytrus, but then Sytrus has a lot that they don't. A synth this good with a (VST usable) host thrown in for $179? A steal, in my opinion. To me, it is very good value for money.
-Stability It hasn't caused a single problem for me so far. I've seen no bugs. I use Tracktion as my host.