The story is fairly simple - J. Edgar Hoover's life from, roughly, the time that the FBI begins, until the time of his death, hitting on some of the bigger and more scandalous aspects of his career.
Now this is always where it gets hard. This is the part where I have to try and figure out how to justify the fact that I like certain elements of the film, while ultimately finding it slightly more interesting than watching paint dry. I liked the acting. The production design was exquisite. The film, though, was about as interesting as reading the wikipedia entry for Hoover. I think the main problem is that it tried to cover so much stuff, that it was always just touching on things, the way that a magazine article would. Don't get me wrong, I get that the point of the movie was to show how diluted this man was, how "in his own world" he was, but it felt like a long, hard trudge through a lot of mud to get to that point, leaving you wondering, by the time it's over, if it's worth it.