Is Daniel Day-Lewis capable of delivering a bad performance? With a win this time around, it'll be an Oscar hat trick for Day-Lewis. And that doesn't include awesome performances in Last of the Mohicans, In the Name of the Father, etc. Any film Day-Lewis chooses to do is usually a quality flick. There Will Be Blood is engrossing but also nothing like what I expected.
The trailer definitely hypes up the action aspects of the film and draws a picture of warring factions when in truth, it's really just about the inner demons of one man. Everything is supremely well done -- directing, cinematography, acting, writing -- but I can imagine how the impact of the film would have been far less if not experienced in an immersive movie theatre environment. If this type of movie is your cup of tea, go, don't rent.
Check out Wikipedia after you've seen the movie to answer the big question you've probably got spinning through your mind. Sure helped settled my mind.