It actually wasn't all that bad; it just begs how much you want to buy into the thinking behind the plot. If you're an atheist? A nonbeliever? Hate religion in general? You'll hate the movie.
Plot summary: 31 years after "the Flash" (which seems to have been an atomic explosion, or several, which had the unfortunate side effect of blowing a hole in the ozone layer as well as killing off almost all the people on the surface of the earth), a man is wandering across the country. When asked where he's going, he simply says "West." This man, later revealed to be Eli, is a machete-wielding badass man of God (of course), who reads his Bible every night, and "walks by faith, not sight" (his words, not mine). He comes across one town where the head-honcho happens to be A) literate, B) A tyrant, and C) looking for a copy of the Bible to get "the right words" to control people. When he finds out Eli has the Bible, he attempts, time and again, to get it from him. Eli continues his journey west, accompanied by Carnegie (the tyrant)'s ward, Solara.
The movie wasn't bad, but thanks to this topic I knew the twist from the beginning. It made it better in some ways, since I knew what to watch for, but worse in others, as I really had nothing special to look forward to.
Let's put it this way. If you like post-apocalyptic, violent movies with a religious bent to them, you'll enjoy The Book of Eli. The acting is nothing special. The music is almost nonexistant (it's like they tried to be "No Country for Old Men" after the end of the world). The action is pretty cool, but pretty spread out. The religious phrases quoted are really common, so nothing too special.
The only thing I did like was the underlying message that religion can be used for good OR evil, and is currently (in our time) being used for the later. Eli is an example of the "good" side of it. Basically the movie says the Bible is powerful; it's up to us how to use that power. Fair warning: you're not likely to get divine protection and awesome fighting skills like Eli.