Dunno... I quite enjoyed it. I loved the design and atmosphere. I enjoyed killing baddies in all different ways. Though the story was better than many FPS games.
I tried to do as much as possible in the game. I explored every single nook and cranny, hacked most machines and vaults, would back-track to pick up ammo, etc. Not sure how long it lasted me in total hours, but it did keep me entertained for a while.
Was I at all disappointed on any level? Sure. I have to admit, I did think it would be more open, with the ability to explore a lot more of Rapture, and not bo totally stuck on rails. However, designing a game like that might take one hell of a long time.
All in all, not a bad game. Not 10/10, but not bad.