There are other reviews here expressing similar views, but I still feel impelled to add my comments. The film is generally well-made from a technical point of view, apart from possibly being too long. The acting is mostly very good, although Kevin Spacey isn't given much space to explain his character's motivation (apparently ambition rather than racism), and Sandra Bullock's only function seems to be as eye candy, which she admittedly manages very well.
At a fundamental level the film's heart is in the right place in being opposed to racism, and I get the impression that it tried to set out to be some kind of definitive treatment of racism, perhaps why so many famous names agreed to be in it. However, it seems the writers bit off far more than they could chew (I haven't read the book, so I don't know how much of that is down to Grisham). The biggest problem, as many others have said, is that it ends up advocating vigilante justice. Aside from the fact that I don't agree with that position, I don't think that such a view actually helps the anti-racist cause at all - it's more likely to be applied to acquit white defendants who kill black victims. At the end the prosecutor invites the jury (and the audience) to imagine that the raped girl was white - but follow that through and imagine the defendant to be a white man who murdered black rapists ... in the end the message seems to be that it's OK to kill someone as long as you hate them enough. It's also convenient that the man killed was presented as totally evil and his guilt was in no doubt, which removes any of the moral ambiguity likely in a real case.
Samuel L Jackson gives an excellent performance, but unfortunately this also undermines the plot - he comes across as someone with considerable integrity, but it's hard to believe that he would be willing to hide behind an insanity plea (and indeed he doesn't, when push comes to shove). We're presumably supposed to think that it's because he faces the death penalty, but in fact at one point he's offered a plea of manslaughter, although if you blink you might miss it. Why not take it, given that he must know his chances of acquittal are slim? Or if he wants to make a stand in court, why plead insanity?
There are less serious flaws too. The medical experts, on whose testimony the case supposedly rests, are jokes - both are discredited for highly implausible reasons, and neither of them offers any real psychiatric diagnosis. The message seems to be that expert witnesses will say whatever they're asked to say, and shouldn't be believed. The fact that the injured policeman supports Jackson is moderately plausible, but still a bit convenient (and what if he had died?) I find it hard to believe that the KKK would march down the street in broad daylight. Other apparently serious crimes (riot, arson, kidnapping, attempted murder) go by without any visible attempt to detect or prosecute them. And the scene where the dog comes bounding back is ludicrous.
My final reaction is to be left feeling rather dirty - as though I must be a racist because I disagree with the resolution. It may be that the film intended to explore different viewpoints and leave the audience to decide, but if so they seem to have forgotten it by the time they got to the end. The film did make me think about my views, read what other people thought here and add my own opinion, so at some level maybe it succeeded - but I worry that it may have re-inforced highly illiberal views in some people, which I hope is the opposite of what it intended.
Final thought - try watching Law and Order, it covers issues like this with much more depth in about 38 minutes of screen time!