I'm a police officer, have been for three years. It's the same as anywhere else, some good people, some not so good. I'd like to think people base their opinions on personal experiences, whether they be good or bad. People (understandably) feel strongly about the police because the things we deal with effect their lives, either as victims, witnesses or offenders. I think that as individuals and officers we should give the people (whatever their status) the best service /treatment we can provide, sadly judging by some of the posts in this thread that's not always been the case.
The police hire human beings, no different from any other employer, in the UK there's a lengthy selection process / training period that's in place to try and ascertain that the people hired are right for the work. Not much else they can do, it's down to the people we deal with day to day to make their feelings known to the appropriate ranking officers or independent bodies if they feel they've not had quality service or been treated shabbily. It's the only thing that'll identify problems and get them investigated/resolved.
I'm proud of what I do but I'm a person as well as a police officer, I understand why people feel strongly. If you've got a valid problem with police complain productively to effect a positive change. Frankly, it'd make my job a lot more pleasant if justifiable complaints were voiced to the right authorities, the right people would be investigated and disciplined if appropriate, save the good officers being judged on the actions of the officers who have let the side down.
I can only speak for how things work in the UK, if you've got a problem you can easily complain and your complaint will be investigated. On a professional note, I don't care what people think of me because either way I'll do my best to make sure they get the best service/treatment I can, that's why I joined after all, but that's me, I can't speak for any other officers and wouldn't presume to do so.
Just offering my perspective as a police officer.