Bioshock 2 was not bad, it was just unnecessary.
The first game closed the story pretty tightly, and had the extra appeal of exploring a weird, mysterious and unfamiliar place. Because of that, the second game was meant to feel less interesting by default. The part that makes you play as a Big Daddy, but still keep all the features of the original makes it be less well-thought (after all, how many times did we see a Big Daddy in Bioshock 1 scavenging for food and money, using plasmids and vita-chambers, and killing little sisters) didn't help.
It also suffered from a lot more repetition than Bioshock 2. The big sister was only introduced to have something over you in the food chain, but it was never explained, it was extremely predictable, the encounters lacked tension and the cycle became tedious soon. At first, I thought she was going to be a unique encounter that would harass me throughout the game and her presence would be explained into the story (like Nemesis in RE3), but it turns out she is never really explained, she only appears when and where I chose to, and there are dozens of those things.
However, the combat is better than the first one and the story gets interesting near the end (not as good as Bioshock 1, but good on its own right), and a mediocre Bioshock is still a lot better than 90% of the FPS games out there. Also, they had the superb Minerva's Den DLC, which is an unconnected, well written story in a compressed Bioshock experience.