How prevalent do you think corruption is in law enforcement agencies?
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.
All of what is said above is true and I have some things to add to it. Many police agencies are now turning to where you have to have a Bachelor's Degree or an Associate's Degree plus two years prior of Law Enforcement experience, such as in the Military. I've looked at many police corruption cases and I have seen that not many of the larger cases have happened recently. In fact many happened in the last century when education was not a large requirement for officers and it was more difficult to track down those linked to the case due to the lack of technology, such as DNA analysis and survellience equipment. Now guidelines and regulations are much more strict for officers which has led to a decrease in police corruption on a large scale. And also, about officers getting rid of tickets for people; the ammount of copies of the ticket itself that is made and for whom they are made for is making it increasingly difficult for officers to expundge tickets from the records of friends.