I have always been skeptical about the value of mounting lasers on firearms. But after talking with such notables as Ken Hackathorn, Larry Vickers, and others I have changed my mind. Lasers give visible feedback to shooters. You can see whether you flinch, whether your barrel wanders, and you can smooth out your trigger technique
There are two main players in government and local law enforcement: Crimson Trace and LaserMax.
Crimson Trace Lasergrips are set for point of impact just like a rifle scope, except you use much smaller tools to adjust it. As the name implies, Lasergrips are built into the grip, which replaces the stock grips of your duty weapon. The grip secures the batteries, on/off switch, and the activation switch. This, of course, means that the grip has to be slightly wider than the stock grips.
On polymer frame handguns such as Glocks and M&Ps, the grip is fitted to the frame or part of the frame is replaced, neither voids warranties. Crimson Trace will soon have grips for Springfield XDs. Crimson Trace Lasergrips are used extensively by government agencies and the military.
Another popular laser sight is made by LaserMax. LaserMax lasers replace the guide rod; the on/off switch is installed in the take down pin or lever on your duty weapon. This means you won't need new holsters and your grip size will not change; for those with small hands this is a major concern. LaserMax guide rods are factory zeroed at 20 yards except for the 1911, Uni-Max, J-Max, and shotgun models, which are adjustable.