This question is responsible for thousands of pages of flame wars on the forums and blogs across the internet, mostly because it’s far too broad of a question to answer with a simple yes or no. Every school owner will tell you that their art is THE most effective art for self-defense because they want you to sign up for classes.
So, is BJJ effective in a real fight? That depends on the situation.
My own personal feeling is that if you train in just about any legitimate martial art, and you work diligently to develop your skill, and you end up in a confrontation with someone who hasn’t trained in any sort of martial art or combat sport, your training will help you. A boxer will out box an unskilled fighter, and a grappler will out grapple an untrained fighter. In this sort of one on one situation, your Brazilian Jiu-Jitsu training will be effective, allowing you to keep yourself safe and then gain the upper hand in a fight.
If you come up against someone who also has trained in some combat sport or martial art, and is much bigger or stronger or faster than you, or is just flat out more skilled than you, obviously BJJ might help, but it isn’t going to be a magic bullet.
Also, if you are facing multiple attackers, especially if they have weapons, I don’t care if you are Bruce Lee back from the dead, you aren’t going to win that fight no matter what kind of fighting style you’ve mastered.
I’ve always said, and I continue to believe, that the most effective martial art for the real world is Track. If you find yourself being attacked and there is any way out at all, your best bet is to run. Only fight when there are no other options because you never know what your attacker is bringing to the table.
So what about all the claims of BJJ giving you the ability to defeat bigger stronger opponents? Those claims are absolutely true, within limits.
If you weigh 150lbs, and become a black belt in BJJ you’ll have no trouble taking care of someone who doesn’t train but is 225lbs and strong as an ox. But if that big guy starts training BJJ, and learns how to defend himself from all your techniques, it’s going to be harder and harder for you to deal with him.
Technique and skill overcome strength and athletic ability, but if technique and skill are equal then strength and speed are the tie breaker.