Personally, I think Cooper went too hard at trying to capture Bernstein's voice and mannerisms, and when that happens you risk giving a mannered performance. Sadly, I think that Cooper tends to fall into that trap, and often I could see the actorly wheels turning. Even with Mulligan too, but to a lesser extent. She has some top notch scenes, but they tend to stand out from other less convincing moments. Somehow the movie kept me at arms length, I wasn't allowed into it the way I'd hoped, and the cinematic flourishes (like the fantasy dance sequence)excellent in themselves, still left me outside looking in.
Funny, because unfocused and aimless pretty much sums up this review. A couple other things which also escaped your attention; the movie is pronounced 'MY-stro,' not 'MAY-stro,' and the central character's last name is pronounced 'BERN-stine,' not 'BERN-steen.'