Well first off, I'd bought Toxic III on sale for $49 a while back, and I loved it to bits. Great for leads, basses, PADS (the best synth I've come across yet!), atmospheres, you name it. Whatever you want, it does it!
That's great Flex, what about Biohazard? Well, it does it even better than its ancestor. Has about a million (ok not that many) more presets than III, and all of them are extremely usable. All I have to do is load up a preset, tweak its filters or something just a bit, and voila! Instant awesomeness.
The sound is lovely. One of the weak points of Toxic III was that its bass was just not quite there sometimes. In Biohazard, this has turned into one of its strong points! It truly is a workhorse synth, capable of any sound you want. Oh and the eternal question..."does it sound like a Virus?" Yes, actually, there's an excellent preset by ToTc Productions that really does sound like a Virus. In my not-super-educated opinion anyway.
Oh and did I mention the GUI? Toxic III was a GREAT introduction to FM synthesis for me, being one of my first synths ever anyways. Toxic Biohazard is actually even easier to use, in my opinion. Now I know how, exactly, FM Synthesis works and it's made life so much easier when working with other FM synths.
Oh, and the addition of the native FL Studio version is a kicker--it allows me to do all the fancy sliding and event editing I want! Stability has never been a problem, Toxic is a rock-solid synth in my VSTi and FLi versions.
Documentation? Honestly I haven't looked at it ever, I haven't needed to!
I haven't used customer support, but the fact that I got Biohazard as a free upgrade to Toxic III speaks volumes for Image-Line.
All this to say that I'm totally in love with this synth, even though I've had it for a while. Its ancestor was amazing and Biohazard is astonishing. Well done, Image-Line and Maxx Claster!