Joseph Wambaugh has written a lot of great books over the four decades of his literary career. My experience with him started in eighth grade in 1972 when I read The New Centurions, a blisteringly honest and terrifying book about the lives of three rookie patrolman in LA during the early 60s. It was easily the most grown-up book I had ever read (my mom thumbed through it and was appalled at the language; yet she let me finish it) and when I got to see the 1972 movie (butchered on NBC in '73 or '74), I had reread it and knew everything the little old ladies with the scissors had hacked out. Even with the obligatory mangling for our living room sensibilities, Richard Fleischer's film is a well-acted and gritty TV-looking version of Wambaugh's great, searing novel.
For the most part, the casting--THE critical step to putting the book on screen--was dead on. Stacy Keach nails Roy Fehler, George C. Scott is a slightly more buff, less urbane Andy Kilvinsky, and Jane Alexander (who is beautiful because she isn't) embodies Fehler's estranged wife, Dorothy). My only complaint is in casting Erik Estrada as Sergio. I know why he was picked--a blonde Hispanic would have confused viewers who had not read the book, but some skilled writing may have gotten the real Sergio across on screen. This is no insult to Estrada. He's hardly on screen, but this was before the excremental CHIPS, the show that ruined his career while making him a household name, and he is quite good for the few minutes we get him.
The problem with The New Centurions is that, since it is designed for mass consumption, it has been rendered more TV cop drama than searing expose of urban policing. It looks authentic, but the color and depth of the images never really fill the wide screen, dooming it to look like it belongs on the small one.
In comparison though, this is a much more successful adaptation of a Wambaugh work than the open-mouthed horror of Robert Aldrich's The Choirboys. That book was even more dark (how Wambaugh was able to make such a brutal novel so funny is still an amazement to me), but the 1977 movie was about as awful--and unfunny--as you could ever hope to miss.
Which, in comparison, makes The New Centurions all the better. Don't get me wrong, TNC is a flawed film, but it is a good one on the whole. I would just, strongly, suggest you read the book--and The Choirboys--first to get the real flavor of one of America's better crime writers (and social critics).