First of all a tip of the fedora to Kevin Burton Smith for writing a thoughtful review of this movie and sparking my interest in seeing it. Hey, you can just click on the link, read Kevin's review, and save yourself the trouble of reading mine.
However, if you're still here, I do a thing or two to say. I liked Silver City quite a bit. I thought it worked just fine both as political commentary and as a private-eye story. And in fact, I wasn't bothered at all by the little mini-lectures that popped up from time to time because the movie was structured like a mystery novel, and the lectures seemed to me to fit right in with the plot. I've read similar things in novels by people like Ross Macdonald and Raymond Chandler. And John D. MacDonald and even Ed McBain. You'll have to see the movie and find out if you agree. Kevin mentioned Chinatown in his comments, and I think the comparison is apt. Silver City has that kind of atmosphere and that kind of ending.
Some people might complain that the movie's too tied to contemporary events and characters (the comparisons are maybe too obvious) and that therefore the movie's limited in its appeal. I don't know why that should be true. It seems to me that even if you had no idea that the characters on the screen were supposed to represent certain current politicians and political operatives, the story still carry you along. The idea of corruption at all levels is timeless enough to interest people in twenty or thirty years, I'd think. But I'm no prophet, nor was meant to be. Have a look at Silver City and see what you think.