Eric Morris has a nice post on the Freakonomics blog: Seeing Red: Why L.A. Needs to Keep its Traffic Light Cameras

Policemen on the street, on the other hand, are vastly more invasive and potentially unjust because they are surveiling you when you are not breaking the law, have the ability to bust you on more severe charges emanating from a traffic stop (e.g. if you have drugs in the car), have fallible judgment about whether you were in the intersection, and have the ability to enforce the law selectively (e.g. racial profiling). If privacy is your concern it would actually be far better to have RLCs, but ban police from the streets. If you concede that it is kosher to have policemen on patrol I see little basis for arguing against RLCs, which are actually considerably more benign.
A major problem for RLCs is common to many public policies: those who are punished know who they are, but the beneficiaries do not. Also, it is hard to point to the benefits of something not happening. People who get tickets from RLCs are often bitter, and can turn into vocal enemies of the program. However, there are hundreds of people walking around today whose lives were saved by an RLC but will never know that they cheated death thanks to a camera. Consider that you might be one of them. Or if you really do hate RLCs, I’d suggest you fight back and teach those money-grubbing bureaucrats a lesson… by stopping at each and every red light.

Alas they are unconstitutional in Minnesota.