Just finished it on normal and I can vouch for it being great all the way through. I will probably start sometime tomorrow on Hard (I can't imagine some of the big daddies on hard...)
I have been playing it as well. My favorite part, though is the satisfaction you get for killing something. I don't know what it is, but the slaughtering of things feels really good in Bioshock.
Give it time. I think the only (=A=) member I have wanted to knife but failed to thus far is Phrogg. Everyone else has a moment of sluggishness from time to time and tastes the blade.
Edit: Got Phrogg today *Marks one more off the list*