One of my all-time favourite RPG's, but make sure you get the Enhanced Edition.
The witcher is different to the other RPGs in that you're playing a particular character rather than creating your own. You do get to choose which skills to level up though.
The biggest difference is the moral choices you have to make. Unlike other games there are no good or bad choices, there are no right or wrong decisions or good or evil choices - just shades of grey. Also the consequences of your choices are not made apparent until some time after you make them, so you can't just reload a save game to change a choice. I really agonised over some of the choices - it really is a case of damned if you do and damned if you don't.