A lot of it depends on the department in question. For example, the PRK "state police" (the CHP) are surprisingly nice people, especially since the 55MPH speedlimit was repealed. These days, they concentrate on pulling over the bad drivers, not the guys doing 60 in a 55. Of course, if they'd just get the slow people out of the left lane...
LAPD, on the other hand, is almost fully corrupt. Even the better guys would be shunned for their attitudes and behaviors in most other places.
SFPD is fairly decent, but is way too controlled by Willie Brown. Oakland PD sees stuff as bad as LA, but on a smaller scale, yet is far better than you'd expect overall. The Alameda and Contra Costa County Sherriffs are run by hacks, but the deputies are decent.
-Troy