This is the most underrated weapon in the game. The flash will completely blind and incapacitate anyone within its radius without risk to civilians. The ROE penalty for the flash is -5, so if there is a friendly surrounded by a ton of enemies and you are sure that you can get all of them, go ahead and flash them. You will have about 15 seconds to kill all the enemies. Keep in mind that this grenade only has a 1-3 second fuse, so be careful if you are going to cook it. Most of the time you should just pull the pin and throw it right after.
This is as dangerous as a fragmentation grenade, because it takes a player out of action for over 15 seconds, leaving them helpless. It also never gives an enemy time to run for cover. If you see one land near you, quickly turn away from the tossed grenade and point your gun at the floor to lessen the effect on your sight a little. There are two things you can do after that. The first is point your weapon in the direction that your enemy will probably come from and start firing a moment after you are flashed. The next is to look in a direction that will lead you to relative safety, such as around a corner, and start running. Continue running until the flash wears off.
An effective maneuver with the flash is to roll it at an enemy. Never cook a flash when rolling. Rolling flashbangs create a huge advantage when you are standing next to the door you want to enter. Instead of throwing the flash and having it bounce to a corner or behind an object where its effects are diminished, roll it through so it explodes just beyond the doorway. There is a good probability that the OPFOR is looking right where it will go off. Also, when rolling a flash you really can't see it when it goes into the room because it is so close to the floor, giving the OPFOR less time to react and turn away from it.
If you toss it, try and angle it so there is an object between you and it, as the short fuse makes it easy to get hit before you have a chance to run, and nothing says easy target like a blind soldier. Especially when they give themselves away with actions like running against a wall.