Finish it. That
was really awesome IMO, and even if the game gets a little worse after that moment, it's still good. Furthermore, you'll find more diaries, who are one of the strengths in the game, you'll find more power-ups and other things that make the 1-2 hours you have left worthwhile.
I really don't get why people say "just don't finish the game" but... well, i'm used to GAF being kinda hyperbolic. Bioshock it's an amazing game and a worthwhile play from start to end, even if it's not perfect. And, personally, it's one of my favorite games.
I don't get why you should stop playing a game you are enjoying.