Laser sights don't REPLACE iron sights, they supplement them. I have the CT sights on my 642. If I can see my sights I use them, in low light where I have difficulty seeing the sights I use my CT Laser Sight.
My LS are activated by squeezing a button on the grip. You don't have to turn on or off a switch.
The best advantaged I've found in using the CT is in dry firing. When you dry fire using the LS you get instant feed back as to what is happening on the target.
Again I think they are a valuable "supplement" to my iron sights, but not a replacement for them.
I don't have CT on my other guns, only on my carry gun.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071