I just finished BioShock 2. IMO, it measures up in every way to its predecessor, and definitely has me looking forward to BioShock Infinite. The basic gameplay is the same, but I really can't complain about that because BioShock's gameplay was just plain fun. The new interaction with the Little Sisters is very cool, if often frustrating: rather than just rescuing or harvesting, you can now have each one lead you to two "angels:" corpses full of ADAM which the Little Sister will then harvest for you. After the second angel, you take your Little Sister to the nearest vent, where you have the familiar options of rescuing (killing the implanted sea slug from the inside and returning her to normalcy, gaining a modest about of ADAM in the process) or harvesting (ripping the slug out of her, killing the child but gaining much more ADAM). The frustration comes from the fact that as soon as a Little Sister begins to harvest ADAM from a corpse, Splicers and (later on) Alpha-series Big Daddies spawn with the objective of lodging a spliced-up foot in your armored ass. Some angels are located in defensible positions where you can use traps to thin the foes' ranks, but most of them are out in the open and/or in places where there are too many entrances to monitor effectively. Expect to die.
One of my favorite callbacks to the first game is the fact that you can now execute the Big Daddy's signature drill charge: you rev the drill, then charge forward at great speed, dealing heavy damage to whatever you hit. Most Splicers can be killed in one hit. Big Daddies... not so much. There are a couple of new Big Daddy variants: the Rumbler, armed with a grenade/rocket launcher, and the Alpha, which are prototypes like the player character and wield the same weapons (either a rivet gun, shotgun, or machine gun). The Big Sisters are outright terrifying: heralded by a scream that sounds like a steam engine being tortured, faster than any other foe, teleporting, plasmid-using, wall- and ceiling-running, giant arm-mounted syringe-wielding maniacs who continue to pursue you even after you die and respawn. The game never really explains where they come from, who made them, or for what purpose; all you know is that they want you dead and are eminently capable of making that happen.
The only real complaint I can level at BioShock 2 is that the story follows essentially the same arc as the original: a rogue influence arises in Rapture and challenges the local visionary, who in the end chooses suicide over defeat. The only difference is that there's no bait-and-switch, no Fontaine-analogue: the Big Bad is exactly who you think it is. Still, the ride is fun, even if you can see the ending from the start.
Bottom line: If you like BioShock, you will like BioShock 2.