It can only be described by one word, and one word only. Epic.
Don't get me wrong, other games from the same world have came close, your Icewind Dales and your Neverwinter Nights', but none have came close to matching the PC-RPG-Gaming perfection that is Baldur's Gate II.
The gameplay, superb. Everything on the screen feels fluid and right. You know what you're doing, no matter how complex it may sound. You know, almost instinctively what to press to suit the occassion.
The graphics, I love them. The almost-painted backdrops, the characters which remain almost two dimensional. The magic effects are what maim the visuals for me, but not by much. The three dimensions on the magical attacks just don't feel right in comparison.
The music is epic, especially the main sequence. On more than one occasion, I've just sat back and listened to that song, over and over. It's beautiful, almost poetic in a sense.
It's value, fantastic, unbeatable. For it's price now, this may be the best discount game you will find. You don't need a super-high end PC to play the game, unlike Neverwinter Nights. This is what, I believe gaming is about. Fun in any form possible, and Baldur's Gate II achieves that, in my eyes.
Of course, I could just be being biased. This game is the second PC-RPG I played [the first being it's predecessor, Baldur's Gate]. This game will always have a place in my heart, and I feel that sometimes, it isn't given the credit it truly deserves.