This depends on what you would count as corruption.
On the one level, I would argue that corruption is common if you count such things as "fixing" traffic or parking tickets as corruption. I believe that practices like those that have come to light recently in New York City are quite common as police do favors for people they know.
But the more serious type of corruption is not very common. I do not believe that is a great deal of corruption on the level of police officers working for organized crime bosses or police officers stealing drugs from the evidence lockers and selling them. This sort of thing does happen, but it is in no way pervasive.