This series and Planescape: Torment stand out to me as major bench marks of good games. They have a intriguing overarching plot, exciting and thrilling side quests. The characters are both diverse and unique from the NPCs to your own companions, you'll find someone you like. And even if you wish to not carry on the story and instead muck about you can. Then be levelled up and powerful ready for next big mission.
The voice acting for such a vast game is amazing and some of the work really shines through. I first played this about seven years ago, and now seeing it again. I know what a gem it was and wish more games tried as hard as this did. And it even had a terrific sequel. So really this game is just a time we won't see for a long time. Most likely it was a Golden Age, thus won't be repeated. Which is why if you read this review, take my word on this being a masterpiece you should take.