I am a strong believer in having an Independent Police Complaints Authority, as we have had for a decade or so.
There is still some way to go as the PCA has very limited resources and often can not fully investigate issues – relying more on reviewing the internal investigation.
If you want reasons why the PCA should be fully independent, have a look at this story on Boing Boing re Florida police. They actually threaten someone who comes in wanting to make a complaint about police behaviour. Rather ironic.
The cop actually chases the complainant out of the station and off the block!!