I figured I'd see the film before responding to this, and I did today, in a good theatre. Honestly, I don't know how anyone expects Bradley Cooper to play Leonard Bernstein without being rather over-the-top...Lenny was a larger-than-life person. It's a biopic, and that WAS the guy.
Personally, I'm not a huge fan of Cooper, but I love Bernstein, and Carey Mulligan. I liked the film a lot. I found the acting was across the board good. Cooper did rather disappear into Bernstein for me, most of the time, and I thought Carey Mulligan had a great emotional arc. I thought that focusing on their relationship was interesting and complicated. Yes, they chain-smoked, and it killed them both. You want to re-write the 1950-70s?
Showing how an artist becomes a great artist is tough to do, though I think they hit on some moments, one in him describing it in an interview with Edwin R. Murrow, I think? (Another great chain-smoker.) And when he conducts Mahler late in the film. (A long take, and very emotional.)
Biopics have to decide what they're going to focus on, and this one went for his life with Felicia. I think it was an interesting choice, and Mulligan held up her end. But they tend to be episodic, not plot-driven, and that can seem boring for some. I'm fine if others don't like it, but I also found the camera work interesting, and I liked the mix of b/w with color. Bottomline: I thought it was a good film, and a good, sincere and serious film.