Blows Oblivion out of the water
Since you're playing Geralt you don't have any choice in terms of your gender or "class" (in D&D terminology). You can extend your sword combo's with additional moves, improve spells, strength, intelligence, gain resistencies and so on. It all works smoothly, though sometimes you wish you'd have the broader D&D choices. On the upside, the character of Geralt is developed a great deal and through lots and lots of voice acting.
This brings me to one of the weaker parts, the voice acting, which ranges from good to rather awful. It's a shame some it is really amateurish, though it seems to get better as the story progresses. Geralt himself is good if a little monotonous.
At times the pacing is a bit too slow, you're going to do a lot of running back and forth sometimes across significant distances. All-in-all I spent about 60 hours finishing the game, so you better have some free time on your hands...
One of the greatest features of the Witcher is the alchemy system. You can extract ingredients from plants and corpses and use them to make potions, poisons and bombs. Since the effects of potions last a very long time (as much as 24 hours) they are especially useful, since you can easily use them as buffs before fights.
Unfortunately the huge selection of ingredients means your limited inventory space gets filled up with lots and lots of junk and you'll inevitably be fooling around with it more than you should. All this could have been solved with an "ingredient pouch" or something. Shame.
Clearly the Witcher is not without its share of problems and small issues, it's a rough diamond but it's got such a great story to tell that it hardly matters. If you consider yourself an RPG fan, you better play this.