Bioshock 2 is better than the original, although my opinion of that seems to differ from the majority. Still, I vastly preferred it. The characters were interesting, made far more sense and were considerably more engaging than the ones in Bioshock, the story had better pacing (not to say that it has good pacing, but at least it doesn't send you on a fetch quest just when you think you're about to engage in an epic confrontation with the big bad), and above all it made Rapture actually seem like a real city. Unlike the first, the focus was more on the people that failed in this giant objectivist experiment, fixing one of my big quibbles with the original in that for a city based on objectivist philosophy, it seemed to look almost exclusively at the characters that succeeded. I don't know why people harp on about the atmosphere of the first game and how the sequel failed to recapture it, I saw it as succeeding where the original had failed in presenting a city that actually felt like a real city populated by real people.
Oh, and there's no big yet obvious plot twist given away in the first 20 minutes of the game (honestly, did anyone not realise Atlas was evil when he was telling you to rip open a little girl?). Sinclaire manages to come across as a decent person in the second game despite presenting the exact same option, and he's only one of the many characters I thought were better than their Bioshock counterarts. Indeed, I found Delta a far more interesting character than Jack, and the final two chapters of Bioshock 2 stand as the best ending of any videogame ever, in my mind.
So yes, get it. It's amazing. The gameplay is better, the atmosphere is better, the story and characters are all better. It's one of my favourite games, and I heartily recommend it.
