I remember reading an interview a while back where he said he was disappointed when Jessie Spencer joined the cast... not because he didn't like him, but because he was allowed to use his own accent
It's not the best American accent I've heard, but certainly not the worst.
Reminds me of something Michael Caine said in his autobiography. When he did a movie early in his career (I think it was Hurry Sundown) he did an American accent and the British press criticised it heavily, yet the American critics were perfectly fine with it... and you'd trust the American critics on something like that.