Great synth. Viper clearly emulates hardware (the Virus TI and possibly Virus C) very well, way better than my expectations. I have a weakness for pads and supersaws. With Viper you can make really phat supersaws and pads, by far better than any other soft synth I've tried (and I've tried many...). You can make all kinds of sounds with Viper of course but the two mentioned sounds are in my opinion what makes Viper stand out.
However Viper only gets 4 stars out of 5 from me because there are a few things that are less good with Viper. It's a slower soft synth, particularly loading it up and browsing the presets (the rest functions well). Despite the measurements taken to optimize CPU and the fact that I have a pretty powerful PC, I still get CPU spikes after running a couple of Viper instances in a project. This can of course be bypassed through bouncing, which is what I recommend doing if you plan on using many instances. These mentioned things makes the workflow less comfortable though. Overall, I'd say sound wise it's 10/10. Performance wise 4/5. The GUI is very good and easy to understand because it's well organized. The only thing a little bit confusing with the GUI at times is at the filter section with the F, M and A buttons. The one you click on will get darker and the one not in use is lighter. I personally think it would have been less confusing if it was the opposite. But maybe it's just me :P.