I'm not a big fan of lasers. I feel that one can get used to looking for the dot instead of at the sights. If the beam isn't on the target, you could loose valuable time (a.) trying to find the dot (b.) wondering why it isn't on.
Is it malfunctioning? Are the batteries dead? Is my aim off that bad? Did I even turn it on? Then you have to seek a sight picture. You have precious little time in a gunfight as it is.
I do believe they have value in training, specifically dry-fire, where you can see where the bullet would have hit if you had actually fired.