I bought the first Bioshock 3 years ago and still haven't finished it. Looking at my save games, it looks like I only played up to level 5 before quitting. The date of my next save coincides with the release of Bioshock 2, which I preordered and paid full price for (but still haven't touched). I finished level 5 and 6 at this time, and then stopped playing again. The next time I picked it up was this past week (in anticipation of Bioshock Infinite), and I'm up to level 10 now.
So basically, I can't seem to stick with this game for more than a couple levels at a time. I only seem to play it when the imminent release of a sequel reminds me of this game that everyone loves, but I somehow never finished. Any other game and I might just write it off as something I just don't get into. But I love shooters, and everyone seems to love this game so much.
EDIT: Just beat it finally. The mood and story made the second half much better, but not without fault.