There's not much I can say about The Witcher that hasn't been said already, so I'll keep it brief and focus on the main three points.
A) An epic story, with choices that make differences and consequences that matter. Mature and dark, with controversial topics handled mostly with care, the tale will grip you from a few hours in and not let go until the end.
B) Combat that starts off horrendously, but much like with the story, gets much better after a few hours. It's still a bit janky, but it rewards patience and preparation more than twitch skill.
C) A beautiful, well-realized world, with plenty of lore and stories to find if you look hard enough. Dozens of optional quests, handfuls of interesting NPCs, and items to obtain.
The Witcher seems like a much older game than it is, because it's a bit janky and feels dated, but if you can stomach the very slow first few hours, you'll open up an exciting ride through a well-realized and fantastic world.
Recommended, but be warned: it is a very long game, especially if you end up doing sidequests or exploring. Set aside a hefty chunk of time to play through this.