Verdict:

It is not that Chinatown is perfect at something, but rather that it doesn’t have any major flaws.
It is a neo-noir from the 70s about an early 20th century private investigator working a murder case for a female client, and it is a good example of that genre, though a little modernized for the modern moviegoer. It has an interesting story, without any major holes, yet complicated enough to be an accurate representation of such a detective story. The sentimental soundtrack is from Jerry Goldsmith, so you know it’s going to be good. Polanski manages to maintain the necessary tension and get a more than serviceable performance from Jack Nickolson and the others (although, given the rumours from the set, I am surprised Polanski wasn’t killed by the actors). The film has a great flow, with a slowly growing tension until the end. Nothing feels awkward or out of place.
Nickolson plays this disillusioned but upstanding detective with just the right amount of Nickolson’s crazy. He is believable as a hardened character who has been around for long enough and seen enough to go a little insane and become cynical, but nonetheless retains a humane integrity underneath the hard exterior. Faye Dunaway plays a hurt victim who tries to hide it all under a tough exterior. None of the other actors mess up their roles. Even a kid who shows up in the movie for about 20 seconds is good at what he does.
The music by Goldsmith is a smooth jazz, filled with instrumental effects that are integrated effectively with the events in the film. His music is a key element in creating the film’s noir atmosphere and tension.
If I had to nitpick, the plot and some of the motivations are not very clear.
The general lines of the story are clear, but some details may escape you. I find that this is a problem with detective stories in general, when the tale goes down one path then down another and introduces minor characters that disappear only to reappear half a movie later and you are left asking yourself who is who and why they did what they did.
The motivations and logic of the villain remain obscure by the end of the movie. Also, for such a steady pacing in the storyline, it ends very abruptly, with a few of the issues still unresolved, and frankly, does not fit the character of Jack Nicholson that the movie had built up until then. Basically, he sort of gives up, even though his whole character in the movie is that he doesn’t give up. It’s like the entire movie gives up.
The message is dour. The theme of the movie is pretty much the triumph of evil. What ya gonna do about it. It’s not exactly a movie for a party.
Overall, this is a very clean and well put together film, but at the end of it, I did not cheer or cry or change my mind about some issue. Chinatown excels only at being a very solid film.