Having played the games and read some of the books I'd give this a solid 8. Might be hard to follow for a common viewer, not that familiar with the Witcher world and characters, due to somewhat confusing time jumps and minor lack of character background explanations.

Personally I loved Henry Cavill as a Geralt. He really captures the essence of his character from the body language to great voice acting. I watched the whole season in two days and can't wait for the second one. This series definitely has its flaws but its beautiful and fun to watch.