TLDR: 5/5 stars if you're a fan of the game/books, probably 3/5 if you're not familiar with the universe already.
After being incredibly worried leading up to the release of this series, I must say I was pleasantly surprised. While there are small things in the series that were a bit off putting, like Triss's hair not being more of the canon chestnut color or just going for the flaming red of the games, or Geralt's eyes just being amber and not the cat-like eyes he's known for, overall the casting was great. I was extremely worried about how Geralt would be portrayed but Henry Cavill did a great job. There were a few moments where the outbursts were a bit much, but it was still way better than I thought we would get.
Why this gets 4 instead of 5 stars though, is I feel like someone not familiar with the games or the books isn't going to have a great time. It will likely be a 5/5 stars if you're already a fan, but there are a LOT of things that don't get explained for someone jumping into the witcher universe for the first time. Granted, fans of the books/game are probably clapping like idiots the first time we see Geralt use Aard to knock some guys back in episode 1, then trying to convince his enemies not to fight with Axii, but none of his powers are EVER explained in the first season. We don't get any explanation of his potion usage, the multiple swords, etc. I feel like a 9th episode, squeezed in the middle somewhere, that explained Geralt's childhood, the trial of the grasses, see him practicing signs/learning about potions, would have gone a LONG way to help people unfamiliar with the universe to understand.
All that being said, as a previous fan, I find myself looking forward to season 2. You done good Netflix.