I don't have one on a pistol but I think they help me get on target faster and with more accuracy. I am talking about an Aimpoint M4 (2 moa) on a Sig 556 rifle. Keeping both eyes open, snap shots at 300 yards on dinner size plate targets is impressive, same with under 25 yard targets.
Be aware that the cheap Famous Maker and NC Chinese crap will be a waste of money down the road. They will not hold their settings and you can not count on them for SD.