BioShock set one of the highest standards in recent memory for a sequel to match, and ultimately BioShock 2 meets or exceeds that standard.
It recreates Rapture in a way that's both original and familiar, which is always a challenge for any segmented work of fiction, whether it's Star Wars, Dune, or The Legend of Zelda. It tells a story that is interesting, compelling, powerful, moving, and shocking, with characters that have more depth than the first.
It is just as fun, nerve-wracking, and exhilarating to play as the original, with the same mixture of horror, exploration, and fast paced combat. On top of all that, it has a solid multiplayer component. As far as sequels go, it doesn't get much better than this.