I love my laser system!
I have an M & P Shield 9mm with a Viridian green laser sight with the Enhanced Combat Readiness (ECR) feature. I love it! Although I think the laser system can, in some cases, provide a measure of intimidation, I would not rely on that factor (but it certainly doesn't hurt). I bought this system with the express intent to provide me a quick and very accurate means of placing an aimpoint on a target from a prone position behind concealment or cover (think home defense scenario).
Another advantage is shooting from a close retention position. On the range, I have held my Shield just above my waistline, or straight in front of my body, and was able to place very accurate shots at self-defense ranges.
Unlike the Crimson Trace series lasers, my Viridian w/ECR is always "active" when I draw (personal preference). Yes, the battery life is shorter, but the laser is only on for very short periods of time when I draw and fire. When I holster, the laser turns off again. In a situation where I want to have my weapon drawn, but don't want to highlight my position, I simply wrap one finger around the front of the laser mount (using a very slightly modified grip) until I'm ready to fire, but I must say I never intent to draw my weapon unless I fully intend to use it.
I personally bought the green laser vice a red laser due to the increased visibility. Since the human eye is most sensitive to light in the 505-555 nanometer range (that's why most tennis balls are bright yellow/green) the spot shows both bigger and brighter both day and night (a phenomenon known as the Purkinje shift). To demonstrate this effect, the next time you are driving down the interstate at night, look at an illuminated sign displaying gas prices in red and green numbers side-by-side. From a distance, the green numbers will look almost twice as tall and bright as the red numbers, even though they are exactly the same size. I simply think this make it easier to acquire the laser spot in very complex backgrounds.
Lastly, as stated in an earlier post, the laser itself (be it red or green) is an invaluable dry-fire training aid. Nothing will highlight muzzle drift more than a laser spot at 15 yards!
Bottomline: Buy it! You WON'T regret it!