I know many Agatha Christie fans have panned this because it is dark, and doesn't exactly follow the story. However, I think you need to remember that this was based from a 23 page short story. It was never one of her full length novels. Shawshank Redemption by Stephen King was also a short story originally, and was fleshed out to a full length movie script. If you're going to take 23 pages and turn it into a movie that lasts for 90 minutes, it's common sense something must be added.
Having said that, I am a huge Agatha Christie fan, and have been for forty years. I have read each of her books at least once, and listened to each audiobook as well. I believe I have watched every Miss Marple episode starring Joan Hickson, who was Agatha Christie's personal choice for Miss Marple, and every David Suchet episode as Hercule Poirot. So, I can relate to wanting the script to match the book. I do not like the revisions they have done on many of her books in recent years. All they usually do is take out many of her tightly written, brilliant scenes and replace them with grittiness, extraneous violence and sex, and ruin much of what made the book great. So, I usually don't watch the remakes through to the end. I normally get disgusted within the first 30 minutes, and give up.
This movie is different. While it's true they did add a love interest for the defense attorney, and add in additional characters which were not in the short story, everything they added was in keeping with the way the characters in the short story would have behaved. There is nothing added that really changes the outcome of the plot. All they have done is taken the characters that Agatha Christie invented, and flesh out what the story might have looked like if it had been 300 pages, rather than 23. Granted, I don't think Agatha Christie would have had Ms. Heilger saying " You f@#$ men". But she quite likely would have her say "You bloody men", which from what I understand comes out to about the same thing.
The movie is brilliantly cast, and acted. The characters are so well drawn and convincing that you almost feel that you are watching real events, and a real trial. The cinematography and lighting, and costume design were wonderful. It looked like you were really watching something filmed in the art deco age. So, ignore the negative reviews. Even if you read the story and loved it this is not your typical remake where the only thing left of the book is the names of the characters. It is true to the story, both in the way the characters are presented and the era is portrayed. Yes, there were things added, but if there hadn't been, the characters would have been two dimensional. I gave it 5 stars, and I think it deserved 5 stars.