My experience is that expansion packs often disappoint.
Beyond the Sword has convinced me otherwise. The proof is that trying to play a vanilla Civ4 game now feels a little empty. I would miss the proper space race, the new units and the fantastic mods too much. This one is definitely worth it!
Very little I can point out negatively, so I will put in a personal gripe, for a feature I was hoping would come back in some form. Please bring the building of a Palace back. That one that in previous Civs allowed you to build it step by step. I know it added little to the point of the game. It was more of a side-game, but it certainly added a touch of personalisation and satisfied the little megalomaniac in us, that makes us play Civ in the first place. Given the graphic and complexity possibilities nowadays that feature could be really good.