What's wrong with speed cameras, in a nutshell:
1) No 'human' discretion. Was exceeding the arbitrary speed limit dangerous at that particular time? A real policeman would normally only issue a ticket if it was.
2) The 'not knowing' for a fortnight or more. You can lose your licence several times over before you're even aware that you've been caught. At one point, it was possible to lose your licence just by going past four cameras in a row, and I think this happened to a few people on the M5 at Bristol.
3) Your right to remain silent is effectively taken from you. If they're going to do that to the motorist, then why not do it to other criminals too? Go on, force a confession out of burglars, murderers and rapists. The crime detection stats would improve dramatically, though I think you'd have a quite few more innocent people behind bars.
4) Most importantly of all, they only address one aspect of dangerous driving. And we wonder why the standards of driving in this country have deteriorated so severely - people now know that they can drive as badly as they like, as long as they don't break the speed limit (or at least slow down for the cameras.)
Speed enforcement is good and necessary, but it should be done by trained police officers and (if necessary) trained civillian staff. The only things that speed cameras have going for them are cost effectiveness and, indeed, profitability.
Okay, not so much of a nutshell, more of a rant. I'll shut up now