(Warning: massive spoilers ahead).
There's alot of controversy about the ending which is the only thing about the movie that even had me wondering if it 100% worked. Upon more reflection I think it does, in a sort of Flannery O'Connorish way. Some critics are saying the whole last 1920s part of the movie is off and that's nuts to me. I always thought Daniel would kill Eli; they were playing the same game from different sides. I expected the oil/blood connection to be made explicitly, even that Daniel would baptize himself with Eli's blood in the last shot of the movie - maybe too over the top? The whole last segment is a brilliant portrayal of what our idols do to us: they turn us into charicatures of ourselves and our appetites. Daniel becomes greed incarnate, literally 'beating the competition' to death. He's the ultimate consumer capitalist, sucking the life/oil/blood out of Eli and everyone in his life.
Something none of the critics I've read comment on is the theme of father-son abandonment that occurs here as in PTA's other work. This is where Day-Lewis' portrayal (and PTA's writing) of Daniel's character is so careful; for Plainview is far from a monster during the first 2/3 of the film. He takes the orphaned boy in during the first act, and even though his relationship with "H.W." is far from ideal, he does evidence genuine affection for the boy which becomes apparent when H.W. is hurt by the gusher and when Daniel weeps after leaving him on the train. Of course, H.W. is only the latest in a string of abandoned sons in PTA's films. When Daniel is forced by Eli to repeatedly scream "I've abandoned my son!" it reminded me of Frank T. J. Mackey's breakdown at his dying father's bedside "I hate you, don't leave me!"
Final thought: PTA is a generation-X film-maker and even though There Will Be Blood is (almost) universally recognized right now as an instant classic, I wonder if it may be resisted by some of the older school film critics and/or the Academy Awards (for everything but Best Actor which almost has to go to Day-Lewis). I thought about this after reading Roger Ebert's semi-critical review where he said point-blank that he considered No Country for Old Men a perfect movie and that There Will Be Blood is not perfect. As I posted about last month, No Country blew me away and I do think it is perfect technically-speaking; but There Will Be Blood not only seeks to accomplish more than No Country does thematically, it does so with a few unorthodox twists - the score by Johnny Greenwood, the over-the-top ending. It's these breaks with classic film-making tradition that mar There Will Be Blood in Ebert's (and other critics') eyes and, at the same time, make it even more powerful to me, someone of PTA's generation. And I guess that's what fascinates me so much about PTA and this movie: he is telling a classic story, even a classic American story, in a pretty straight-forward way for the most part and doing it with stunning excellence. Yet, even in the midst of working within the epic film tradition, he updates it, not only in terms of style (score, ending, etc) but also in terms of theme (consumerism, father abandonment).