People's opinion about American police is not in any way different from the most commonly spoken things about the police department in most countries. While American police are quite effective in some areas, especially in reducing certain crimes in the society, however, American police have been called all sorts of names. There have been numerous cases of Human Rights infringement, especially towards the black.
You just find yourself getting deep into more problems just because you are black. I've heard so many black people saying they were arrested just because of some alleged crimes that cannot be proved by the police. This is getting really frustrating, especially when you know that you can be arrested for nothing. This has made a lot of people concluded that this abuse of power by the American police to the black is just another way of encouraging racism. I think aside from this problem, American police are really great at solving crimes.
This is a very controversial question and the answer will vary depending on who you ask and where they live. It also depends on the definition of "good". If by good you mean effective at their jobs, American police are on par with any other police force in world in terms of preventing crime and bringing criminals to justice. If by good you mean moral, that is an entirely different conversation.
Many Americans would argue that the American police are biased against certain races. This type of bias or racism can play out in racial profiling, police brutality, or more formal programs such as Stop and Frisk.