Laser is very direction with a minimum scatter. Because of that, a detector won't provide nearly as much advance warning with laser as it can with radar, although under the right circumstances it certainly can give advance warning as the guys above pointed out. But I do want to reiterate that it is most effective when combined with either active or passive countermeasures. They will give you a few extra seconds to scrub off speed even if you are being directly targeted.
BTW, my research indicates that radar detectors are legal for use in private vehicles in California
, but mounting them to the windshield can cause problems if the police decide that it is obstructing your view (I'm not a lawyer, this isn't legal advice). It seems that Cali may have stricter laws on stuff like that than most other states. But, enforcement is up to the officer, who may not care if a GPS is stuck to the windshield but may take umbrage to a radar detector on suction cup mounts. All I can say for certain is that I've never been ticketed for my detector when traveling in Cali. Of course, as I said, I'm not a lawyer...
Edit: Here are a couple of links for detector laws: AAA