I re-watched this movie today for the first time in years and I have to say that it never gets old. Along with Taxi Driver and On the Waterfront, this is one of the only truly flawless films I can think of. The moody cinematography, the crisp and alive script and the acting are all first rate. I love Polanski's direction, never too flashy but still stylized enough for him to make his mark. Its also very economical, there isn't even a shot that is wasted. The high point has to be the acting, John Huston as the creepy old father is as revolting as he should be. Faye Dunaway in probably her best performance ever, you feel a lot of sympathy for her throughout but you never completely trust her. Jack Nicholson steals the movie though, as the wise*ss, well dressed, private investigator Jake Gittes. This is typical Nicholson in a way but before he became an over the top charicature of himself. He's as obvious as he has to be in some scenes but enjoy the subtlety of his performance. The bedroom scene with Dunaway is killer.
So yeah, great acting, directing, and an ending that will stay engraved in your brain for years. Definitely a top five movie.