The user interface is very good, everything is clear and in a sensible order and it looks the biz. All sliders etc. can be controlled via a midi controller easily by right clicking on the slider, selecting assign, and twiddling your knob.
The way the VSTi integrates into your host is the best I've seen with any synth. When you load it up you get a Vaz 2001 box and clicking on it loads Vaz 2001 into its own window with its own scroll bars. This means that you are not limited to the size that the host permits/has available.
The sounds are very good as you'd expect from any top class synth (great leads, pads, drumsounds, bass, effects, phat sounds, thin sounds etc). You can recreate great sounds from all the classic analogue synths (e.g. Moog etc.).
Features are great! 16 synth multi-timbrality, 16 voices per synth, a mixer, a sequencer, can use DX plugins, VST plugins, lots of routing oiptions, up to 3 LFO's, up to 3 oscillator, 2 assignable ADSRs, lots of filters to choose from, arpeggiator (which is very very good), built in FX pludings (delay, Phaser, Flanger, Reverb, Chorus, Autopan) slide function, portamento, can use waveforms and modulate external audio. The list goes on...
VERY IMPORTANT: Vaz VSTi/DXi is a VSTi host itself so you can load up VSTi's/FX into each channel in Vaz's mixer. This is great if you use Cakewalk products (e.g. SONAR) because you can use VST Instruments and Effects in SONAR through the Vaz Mixer.
The forum is very useful and you always get a response from the Vaz team within the day. There are loads of top tips and user patches/banks too on the site. If you post bugs reports they are always fixed in the next release.
The preset sounds that come with Vaz are available on their support site and are very usable.
I have been using this synth for about 2 months and to date it has never crashed - it is very stable