Mercury hasn't got the sexiest look on the planet, but it's functional and the controls are well sorted. I tend to be using Mercury for the same sounds over and over again, but it's just so good at them. Either as a reso bass, or as an attack synth or as a brutal lead sound. It's great, say, if you have a rock band and the chorus just hasn't got enough drive. It's not the most versatile animal, it has it's own characteristic sound, but it is very good at what it does. It comes with a great number of presets, all of which are well programmed and useable.
Its price, unfortunately, seems to be the shadow of past times and, in my view, should be rethought.
The single most annoying thing about Mercury is that it 'forgets' presets when used in Logic Audio. Also, if you zap through presets, select one, then modify it, for some obscure reason you can't go back to switching through the presets, as the controls to do so are then inoperative. For logic users, Mercury is a nightmare simply because of this little, yet devistatingly irritating bug.
Still, Mercury is a good synth, and it is truly an animal of a kind, with a strong character and good useability.
You won't find it anywhere... given that it was not free. All the copies you will find (except in warez sites but it's not at all my cup of tea) are locked copies used as a limited demo as long as you don' register.
Here is the Wayback Machine copy of the registration page of Mercury-1: