I've seen Crash because of the postive remarks in this thread last week, and I must say I'm not so enthusiastic about it.
For a Hollywood film it is certainly way above average, but for an independent film, definitely below.
What was the worst thing in the film was the terribly pathetic score, which got on my nerves more and more, and which almost succeeded in ruining one of the best scenarios that has come out of the US this year. 
This said, the scenario, and especially the dialogues are the films main support (though they too have some flaws), making very good performances by Matt Dillon, Thandie Newton, Don Cheadle and Sandra Bullock possible. In fact there isn't one mediocre performance in it. 
But again one big problem is the flashy pacing and cutting, which are more of a show-off than a thorough reflection. Some moments appear pointless and "overdirected", and there are numerous deja-vus, as one remebers almost the same things made in a better way before (e.g. 21 Grams, Mystic River, Short Cuts, Magnolia, etc.).
The film doesn't offer any new insights, but is (at least from my pov) right in the middle of current invents where we observe that the american "melting-pot" concept doesn't work.
I can't add much to the positive things that have already been mentioned, let me tell you only that the film had one great moment, in fact for me one of the best emotional experiences in film in my whole life. The scene where Matt Dillon rescues Newton from the car-wreck is in avery aspect a phenomenal achievement, one that should be studied in every filmclass. But it only works so good, because of the scenes and the atmosphere we've experienced before. This scene is the films climax, and I'd recommend evrybody to see the film, if only for this.
There is also a great scene at the end, when one of the immigrants is shown standing in a videostore in front of a huge display of goods, but I remember the exact same scene from another film (maybe somebody casn help me here), but nevertheless this one shot conveys maximum complexity in minimum time, and in the end says more about our world's problems than could be expressed in words.
Despite myself praising certain aspects, the film as a whole doesn't say anything profound or new, that hasn't been said in a better way numerous times before. For me a film isn't automatically good, just because it makes one think. I think the pressing actuality of its topic and the need to discuss the things mentioned in the film in our society (which constantly tries to suppress them) makes a lot of people see quality#s in the film it actually hasn't.
Yes, the film is important, because it allows a big audience to discuss things that should have been adressed much earlier, but this doesn't make the film AS FILM much better than the mediocre Fahrenheit 9/11. But at least this film knows abouts its faults and it tries to show people in an honest way.
It's a good film that has raised more pressing questions than any other I have seen this year, covering a wide range of issues in a good way.
My rating **1/2 / ****, and a "go see it" to everyone who wants to understand our western societies (and especially the US's) problems, and have them presented in a compact way.