This is my favourite freebie VST, and being a man of few synths this means a lot in my book. I spent a lot of time downloading demos and tweaking synths to know exactly what I want, and my veredict is that ReZ could easily be your workhorse synth. The usefulness of Rez comes from the fact that it's very easy to get good sounds without it getting on the way of your creativity. It's a fantastic synth for composing and prototyping.
Being a monophonic single-oscillator synthesizer doesn't limit it as you would think: quite the contrary, you can get a good variety of sounds without difficult. The sound is very good and is showcased by the presets.
Soundwise it's very good too. The effects are the big secret here, just like old synth string machines. The sole oscillator offer enough variety and tweakabilty and the filter is really outstanding for a free product (actually that's where Rez got its name from). My favourite thing about it is the Drive function used to fatten up the sound, something I miss on some commercial synths. The arpegiattor is very easy to use and fun too, and the graphical envelopes are the icing of the cake.
I am giving a 10/10 on all categories simply because it just works perfectly and does everything it promisses (and more).