Laser Jammers:
You have to mount devices to the four corners of you car, or around license plates. First off, this is tacky and a major effort. Not to mention it needs a control computer under the hood and dash, and lastly a display unit for inside the vehicle. There is no true all in one piec laser Jammer. They are bullshit. Think about the fact that you have block the laser with pinpoint accuracy. One single box is not going to do this. The laser can point anywhere. Usually the front are rear bumper, or more specifically the reflective license plate.
Second, the cop is just looking for a reflective surface to reflect the laser. If he doesn't aim in the vacinity that the four jammers are mounted, they are useless. The Jammers aren't going to protect the entire perimeter of your car. The most common places are the passenger and driver side of the front and rear bumbers. This will pretty much cover your front and rear.
The cop can still ping the side of your car, or any other surface. So its' a risk to install these, may or may not be worh while, and they are not entirely accurate. As it was mentioned before, they just temporarily scramble the signal, the PIG will eventually get a lock on you.
The idea is to give you a little more time to slow down.
The point was also made that laser detectors are useless. This is true. By the time your sytem detects laser, it is too late (imagine the speed and focus of light). Before laser, the bands were so wide and unprecise, you could detect them several seconds before they were exact enough to pick you up. Laser is too fast and accurate for this.
If you want good accurate reseach on this topic, google "Radar Roy". He is an ex pIG who tests these sytstems for a living and reports. There are several good testimonials and reviews there.