Arguabley, this game is better than the Baldur's Gate series that it is closely related to.
It is one of the most surprisingly original games I have had the good fortune to play. It is firmly based on the D&D RPG games, but it takes the whole genre and turns it on its head. Rather than creating a character and then trying to play as if you were that character, you play how you want and the character is built according to your actions and choices.
It has the greatest set of NPCs in any game ever, and I've never felt as attached to my party as I did in this game.
Whereas Icewind Dale took out a lot of the plot and roleplaying and emphasised the exploration and fighting, this game takes out a lot of the exploration and fighting and emphasises the plot and dialog. Charisma and intelligence have played such an important role as they do in this game.
If you liked Baldur's Gate for it's depth and originality, then you will think this game is possibly the greatest ever.