OK, I'm in the minority here, but I liked it.
I also like period pieces and I like this period, so...this might be a bit bias.
But it has Scarlett Johansson in it and she is always watchable even when she is at her worst.
And it has Josh Hartnett and I really do like him as well, and I like him in the sort of William Holden sort of he tends to do well when he does a voice over kind of way.
Except he isn't dead in a swimming pool at the start of the film. However, Mia Kirshner is certainly dead at the start of the film and she's one of those actors that usually does a great job despite being constantly over-looked.
What you have is a film VAGUELY about The Black Dahlia, VAGUELY about a couple of buddy cops with a secrete, VAGUELY about a love triangle, and VAGUELY about the period....and that is why it didn't sit well with so many people. It was Vaguely about a lot of things and never exactly about any of them.
However, there is enough mystery to it to hold my attention, enough style to make it cool and fun, and good enough acting to make it believable.
In other words, it still entertains despite the mess. Because of that, 10 out of 10, there is a lot wrong with the film, but ultimately it achieves its goal.