While it didn't quite live up to its predecessor, BioShock 2 is still a fun, immersive, and overall enjoyable experience
In this installment, you play as a prototype Big Daddy, referred to as "Delta". You have all the amenities of a Big Daddy (big Drill and a diving suit for underwater exploration) while having a less bulky and more maneuverable body.
The thing that makes BioShock 2 stand out isn't so much it's plot, but rather its unique and (in my opinion) upgraded gameplay. The creators took what was there in BioShock, and added several new features to enhance the experience. For example, you can now utilize both Plasmids and weapons to take down your foes with a smoothness that the first lacked. No more awkward and time-consuming switches from an Electrobolt to the Machine Gun, or Incinerate! to Pistol. It's all there at once, for you to combine the joys of.
Another example is seen in the "gathering" you do with the Little Sisters. Instead of just "Saving" or "Harvesting", you Adopt the Little Sisters you come across, and they can lead you to corpses that contain ADAM. When you let her extract the ADAM, you will be swarmed by Splicers. This creates an exciting aspect to the game, as you have to protect the Little Sister (and yourself) so you can obtain your ADAM. If a Splicer gets to her, the extraction is halted until you save her. It is often advised that you set up traps and the like to better protect the area around the ADAM-rich corpse. Little electrified wires, proximity mines and rivets, and the Plasmid "Cyclone Trap" all help make the protection of your little ADAM-sucking child easier (though, with the rate and numbers of Splicers coming for you, the traps will be used up quite fast).
While on the subject, the Little Sister/ADAM gathering system has changed quite a bit in this sequel. You still must get the girls by killing their Big Daddy protectors, but you only have one option at this point: Adopt. From here, you may choose up to 3 paths to follow: use her to get the 2 "gathers" from corpses and Save or Harvest her, or go straight to a vent and Save her, or go to said vent and Harvest. You get more ADAM by gathering first, but it's purely optional. Again, as in the first, the moral dilemma is ever present, with the Save or Harvest options.
And while you have an Adopted Little Sister, you may worry about "dying" and losing her. Well, luckily, should you die, she'll be right there outside the Vita Chamber, waiting for you to pick her back up. If this happens during the gather process, however, you must start it all over again.
As for the storyline, it's inferior to the first BioShock's. While it is still captivating and engaging, there is a lack of atmosphere to it. You spend the whole game already knowing what you're primary goal is, so the whole idea of surprise is somewhat missing. There are the little twists and turns along the way, but nothing that will completely shock you or put a sudden wrench in your progress. And unlike BioShock, there isn't a major, awe-inspiring twist to change the entire game's dynamic. I will not spoil anything, but there is still a somewhat "major" twist in the plot. You may be a bit disappointed by it, if you're expecting something crazy akin to the first.
I think one of the things that made the story feel lacking was that for most players, having played the first BioShock, the curiosity of a utopia-gone-bad scenario is gone (which I found utterly fascinating). Since we already knew how this place became a virtual hell-hole, all we were left with for plot material was the back story of your character, which I found to be less appealing than learning about Rapture's destruction and your part in the current setting of it. Then again, that's just my opinion, and I'm sure there are many who have played the game and would disagree.
Lastly, the new multiplayer feature has made BioShock 2 a game you can spend even more time playing. It is generally like the multiplayer found in most other first-person shooters, but with the gameplay found in the BioShock series added to it. Game modes are like other common game types (Deathmatch, Capture the Flag, etc.) except they have a BioShock twist to them. It's a pretty fun part of the game (when isn't online shooter action fun?), but not really important unless you want to get every achievement/trophy the game offers.
Overall, BioShock 2 doesn't live up to BioShock, but it's still a very exciting and fun game to play, and is a must-get for fans of the first title. Otherwise, you might be disappointed, and in that case I suggest borrowing it from a friend or renting it first.
Thank you for reading :)