When I still lived in California, I watched a documentary on KQED investigating the subject of police brutality in SF. According to that source, of the entire SF force, only a handful individuals (named by names), a small fraction, indeed, were responsible for virtually all citizen complaints of brutality. The source also stated that these individuals had a strong backing from the law-and-order political quarters, so the city repeatedly paid compensation to the victims rather than firing the offending individuals.
I think this situation is fairly typical of most police departments across this country - the abuses are rather sporadic, and committed by deranged individuals who may or may not have some backing from the powers that be, but who definitely do not represent the police force as a whole. Needless to say that such abuses receive the amount of publicity in the news coverage that normal police work never does.
I may also add that I am trying to minimize my contact with the police as much as I can, because I believe that such contact seldom produces any good. Having said that, however, when I am on a city street alone at 1AM I feel much safer seen a police anti-crime unit than, say, a bunch of boisterous young males.
Wojtek