Paul Thomas Anderson has made some very good, critically acclaimed, and at least moderately commercially successful movies--"Boogie Nights", "Magnolia", and "Punch Drunk Love" are the famous ones. But "There Will Be Blood" is his masterpiece.
This movie is about a lot more things than oil. There's family, religion, greed, trust, betrayal, danger, and false pretenses. It's ironic that the hero (or anti-hero) is Daniel Plainview, since nothing is quite what it seems (and Daniel Day-Lewis is excellent in the role). As I said, it's about a lot of things, but if I had to sum it up in one sentence, it's a scathing religious epic with Oil playing the role of God (God is also up for the role of God, but Oil wins).
But you can read plenty about this movie elsewhere, so I won't write more of my thoughts. It's a dense 2 hours, 38 minutes, and it'd take more effort just to organize my thoughts coherently.
and But I do want to mention a new trend I've noticed (that's probably not really that new). It's a carbon neutral movie. I know this has been done before, with "Day After Tomorrow" and "An Inconvenient Truth", but recently I've been noticing movies proclaiming their carbon-neutrality in the end credits. This is the third I've noticed, the first two being "No Country for Old Men" and "Charlie Wilson's War". I did a quick Google search and I could find articles about movies going carbon neutral but no comprehensive list of which movies have a carbon neutral statement in the credits. That surprised me. I'm sure I just didn't look hard enough.