I am close to completing 1999 mode which is my 3rd run thorugh this game. I would recommend that anybody who buys this game does the first run through on Easy but checks out some of the excellent web-sites for tips and tricks. This game is involved. You need to understand how to use the vigors (magic/super powers) coupled with the best set-up for the gear (additional upgrades collected throughout the game), try to carry appropriate weapons and search everywhere for goodies.
In my first run through I got very frustrated because I approached the game as just another FPS - this is not the best approach. As I suggest in the subject, you almost need to do the easiest level first as an apprenticeship just to get used to the game dynamics and options. In my second run through on 'hard' I really started to plan the best strategy for upcoming levels - obviously you either need to be following a walkthrough or have already completed the game once.
1999 mode is difficult, very difficult. I am not a 'highly skilled' gamer but I am managing to get through this mode.
The story is excellent and there are a number of interesting twists during the unfolding.
I rank this as one of the best games I have ever played now that I am competent in the gameplay, if I had written this after the first run-thorugh my opinion would've been different.
Now this game is in the bargain basement, it is excellent value for money. I have also bought the seasons pass to get the DLC episodes and some extra goodies (the extras help with 1999!).
It is sad that there are no plans to go further with Bioshock but it is an excellent game, too good to miss.
UPDATE: I completed 1999 mode this weekend :-) The last battle was not too bad using the tried and tested techniques (see IGn for details) worst bit was the Siren battle but patience is a virtue! This game is brilliant!