Alot of these guys who have responded obviously have not experienced a *good* laser. I saw a Crimson Trace laser on a friend's Beretta 92fs, and although I think it redundant, we used it in bright daylight at an outdoor range, and was able to see it clearly at the 50 yard mark. Many of the less expensive lasers use a longer wavelength and are rather weak. If you are set on a laser, make sure to get one with as short of wavelength as possible (this means more money..usually several hundred $$).
I do need to remind you..a visible laser, although "cool" and "hollywoodish", isn't very practical on a firearm, because in order to use it, you'd have to forgo using the supplied iron sights. If you start using the laser exclusively, you would develope some pretty bad habits. What if your laser ran out of battery at an inconvinient time? You'd have to transition back to iron sights (and if you didn't practice with irons, you're at a serious disadvantage). This goes for carbines, handguns, shotguns, what have you. Visible lasers also let you get away with bad posture. You can aim without having to bring your gun up, and if you practiced this way, you'd never be accustomed to using your iron sights.
However, there are certain special law enforcement applications that I could think of, that could definetely benefit from a visible laser and forgoing the standard sights.
1) Firing a pistol one-handed around a tactical/riot shield.
2) Sticking your arm out of a car and firing it forward without actually sticking your head out of the car.
Of course, I don't think I, or any other civilian would be doing this for any reason, but for law enforcement or military reasons, I could see this happening.
Bottom line, don't use a laser on any gun. Instead, get the best iron sights possible for your pistol (I prefer Novak low-mount) with a tritium dot on the front sight only (if tritium on rear, make sure it's a different color). Practice hard with it, pan left, pan right (no more than 90 degree angle, so you don't muzzle anyone), and practice engaging multiple targets in different locations. Get used to what the sights look like when they are *on target*.
For a rifle, red dots are all the rage (they don't project anything forward), and they do require you to at least bring the rifle up so you can see the dot, so you will still be practicing good posture and will regularily see the iron sights.
-Jim