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Having had a stun grenade set off six feet in front of me during a police training program, I can tell you that they are indeed very effective.
The flash is negligent, so no, they don't white out your vision, unless one gets tossed into a dark room you've been standing in for an hour. The bang, on the other hand, is extremely loud, and can easily disorient you, even if you're expecting it. But unless it goes off right next to you, it's not going to make your hearing go out and your ears start ringing.
TL;DR Yes it's effective, but the effect it has on you is completely different from what is normally portrayed
