I doubt anyone knew him outside the Slavic parts of Europe (Poland, Czech, Slovakia, Russia maybe), but I think he now receives growing attention thanks to computer games Witcher and Witcher 2. I won't write about the Polish movie adaptation of Witcher, because that was, plainly speaking a disaster. But all the Witcher books are worth reading, especially the earlier short novels. I think he can do short, focused stories much better than epic sagas. 

However, I wanted to recommend to your attention his excellent trilogy about the Hussite movement in Middle Ages Bohemia:

Narrentum

God's Warriors

Lux Perpetua

Have anyone read them by chance? Is there an English translation?

It was an excellent read for me - wonderfully researched historical background, frequent Latin quotes, interesting, non black-and-white characters, witty anachronisms and situational jokes with prominent historical characters (Jan Guttenberg), excellent, vivid battles, brutality of the inquisition and secular ruling class. What I could do without is Sapkowsky's peculiar magical eroticism, but I guess if you read about people like Crowley, it may not be too far-fetched. Sapkowsky is no Umberto Eco in terms of historical accuracy and depth, but he still pushes fantasy several levels above anything else this genre has to offer (if I am wrong, I will be glad to read something fresh. So far, most of the traditional "fantasy" books I tried offer very little fantasy at all, just mixing the slightly moldy stew of traditional props like pointy hats, fireballs, dragons, evil artefacts and B class heroism).