I'm a big Ellroy fan and having read the Quartet I found this adaptation to be lacking on several levels. It's just too simplistic and lacks many of the subplots in the book. Saying this is pointless I know but books with dense subject matter don't make good films.
But I could take issue with the casting. I have never rated James Cromwell and he's thoroughly unfrightening as Dudley Smith, a man who is truly monstrous.

Also this film has the twattish Russell Crowe in it and I can't bear him.

LOL..you just reminded me of the South Park Russel Crowe episode -

"Born in New Zealand in sixty-four
A hot-headed actor named Russell Crowe
He loves to act but he loves one thing more:
Fight-in' Round The World

He fights his directors and he fights his fans
It's a problem no one understands
If there's two things he love it's fighting and,
Fight-in' Round The World

Makin' movies, makin' music and
Fight-in' Round The World!

Russell Crowe!

Tugger's whistle's blowin'
Means we must be goin'
No more Russell Crowein' for you.

But now don't you start to whine
I'll see ya again next time.
'Cause there's plenty a-more a-foightin' left to do.

Makin' movies, makin' songs, an' foightin' round the world."

I haven't read the novel, but I loved the way the movie was structured -- two guys who were at each other's throats the entire movie team up at the very end to take down their mutual enemies, and in a shootout, no less. Cool.

I try to avoid reading novels until AFTER I've seen the film. Of course the plot has to be somewhat simplified - hell, the last Harry Potter film had to be simplified, and it's a fricken children's book! Books and films are two different mediums, and no film will ever exactly match the source unless the book is very short and very cinematic. This is a great film. I have no doubt the original novel is a great novel as well, and I may read it. But I sure am glad that I hadn't before seeing this film.