The Witcher is my favorite RPG ever. Mostly because of it's story, characters and setting; I've played no other RPG that comes close to it in terms of those three things. If I'm gonna play an RPG, it needs an awesome storyline, and The Witcher fit the bill.
The Witcher 2 took everything great about the original and basically turned it up to 11. It's also extremely different (combat system being the most notable change). Someone above me called it "hack 'n' slash"; while this is partly true, the combat in TW2 is NOT as simple as hacking-and-slashing. It's actually extremely strategic and methodic. The game is very unforgiving, and if you run into a group of enemies waving your sword around, you'll die very quickly. You MIGHT survive if you play on easy, but otherwise, expect to be slaughtered.
There were some odd design quirks in TW2 that were a downgrade from TW1, but for the most part, TW2 was an amazing sequel. Seriously, the characters, the dialogue, and the storytelling were done so well, I dare you to find another RPG that does it as well as The Witcher series.