Just finished watching the entire first season on Netflix and here are some thoughts:
They managed to to partly recreate the lore of the books in GOT style, though the video game did it so much better.
Yennifer back story was invented from scratch, which was good enough to explain her appearance at the point where she meets Geralt for the first time, but the execution could be much better.
The main antagonist of the books series appears only in the last episode (or one before last), and he does not fit the book description. He's suppose to be much stronger.
Episodes vary in their quality overall. While for the first one I could give 9/10, especially for that amazing sword fight, the second one is dull and it was clearly less invested then the pilot (5/10 at best).


Conclusion: the show isn't bad and in few episodes there were glimpses of that amazing world we all know and love, but it needs improvement. Waiting for the second season and hoping they would improve the pacing and give the series more of the Witcher 3 look and feel.