It's the ending that disappointed me. Charlie Kaufmann sure can write, and he skillfully puts up a number of balls in the air, but then, just as his problem of "how to end the script" is oh-so-self-referentially depicted, things falls apart: the twin brother is summon to help, and sure enough, immediately afterwards the movie turns into an actualization of Donald's (the brother's) cinematic cliches -- car chases, shooting, and an awful confession scene (the two brothers reminiscing to high-school on the tree stump in the swamp. ugh!)
So, okay, it's clever how the movie turns into crappy action the minute Donald is recruited to help (and it gets extra clever points for having a "Donald Kaufmann" credited in the actual film title sequence), but ol' McKee specifically commanded that Charlie has got to give us (the audience) a good ending. The action scenes and the Hollywood ending (he grows, he dares to confess his love for the violinist, he is reconciled, the end) do not constitute a good ending. Again, you could claim he's following McKee's advice to the letter: the character grows/changes, happy end. But on the external level, i.e. the level of the film actually shown to live audiences in movie theaters, the film pretends to escape the Hollywood paradigm, only to come full circle in the end, and that's disappointing.