It's an excellent game that does one thing better than any other recent rpg has done in years. If that one element interests you, then you'll likely love the game. If however you don't find it that integral an element, you probably wont be terribly impressed with The Witcher.
Consequences.
Sounds simple, but let me put it this way. In most 'Bioware' esq rpg you'll occasionally stumble across situations where you're forced to make a choice. Whether it's a choice between good or evil, rude or polite, or whatever else have you it tries to imbue the game with some sense of player interaction by allowing the player to decide the outcome of certain situations. The Witcher does this, and it does it
extremely well.
Don't just take my word for it either, Rock Paper Shotgun
(Easily the best pc gaming websites out there) wrote a massive article on it just recently.
Take a look! No major spoilers and it explains things much better than I could here.
As for the setting and characters, having read some of the books the game is based on I loved both. People accuse The Witcher of being sexist, but that's simply an element of the setting. It's not a pure and innocent setting, the Witcher books and by extension the game were originally written as a subversion of those fantasy tropes. It's dark, gritty, rude, and indeed sexist as hell. The Elves as an example rather than being a pure and noble race of tree dwellers are an oppressed minority clinging to what scraps of land they can while violent extremist elements fight a brutal war of aggression against humanity. Basically like Dragon Age, only The Witcher did it first and did it much, much better.
I also happen to kinda like the combat. Oh no denying it's rough at times, but playing on the harder difficulties you're really forced to use potions, weapon enhancements, sigils and bombs to their full potential if you don't want to be cut down within seconds. And I like it when a game forces me to think. The bestiary and journal is excellent as well, being able to read up on different types of monsters to learn their weakness is fantastic and adds another layer of depth to the games setting.
Again, not really surprising considering the game is based on a series of books. Nice to have regardless!
Looks pretty good and has a fantastic soundtrack to boot as well. Voice-acting can be a bit spotty at times, but if it really bothers you there's the option to also play the game in its native language with subtitles, which is a brilliant little touch.
Game isn't perfect by any means, while I generally enjoy the combat it can also be pretty frustrating sometimes and the games got a lot of those moments where you get the feeling they wanted to do something great but had to settle for mediocre because of funding and engine limitations. The beginning part is also a bit tedious. I really dislike that damned swamp in chapters 2 and 3! It's a neat setting at first and looks pretty good, but the place really gets annoying when you spend so much time there and those damned drowners just keep attacking in wave after wave... Eug. But for the price GOG is offering it at? Amazing value.
And I really, really love any game that forces me to make decisions that feel not only like binary good/neutral/evil choices but also have long lasting and unexpected results.