I thought it was just awful and so melodramatic. Thematically, it just didn't make sense, and the acting by the leads was so stilted... it reminded me of an amateur college production whenever Kate and Leo had a scene together. There just seemed to be something stopping them from really getting into the characters. Indeed the only time their acting seemed even remotely real to me was from the final breakfast scene onwards, but it just wasn't enough.
I like Michael Shannon a lot and I'm glad he's getting this nomination, because it broadens his horizons, but he gave an underwhelming performance. The role was a showstopper in the book, and pretty much anyone could have impressed with it, but I found his acting hammy (that obvious thing he did with his mouth and the squinting!) and unimaginative. I've seen two far superior and more creative "crazy" performances this year - Jason Butler Harner in "Changeling" and of course Heath Ledger in "The Dark Knight". I thought the best performance actually was given by Kathy Bates, who I usually dislike. A key scene of her character's was left out of the film (which is sad, she would have gotten an Oscar nomination had they left it in) but I thought she was excellent with the role of a woman desperate to hide her inner sadness. She gave a consistently surprising, subtle performance that was the complete opposite of Shannon's blunt raging. The supporting performances ranged from good (I liked Kathryn Hahn's comic stylings and the guy who played Bart Pollock with great charisma) to awful (Dylan Baker, a fine actor, is downright embarrassing here).
The marvelously cynical ending was one of the few things I really loved about the book - but there it worked because the overall tone of the book was so caustically snide. With a film this painfully earnest, you can't tack on an ending like this and expect it to work within the context of the picture. The entire satirical tone of the novel was completely lost in its translation to the screen.