BJJ is effective up to a degree. Reasons:
-takedowns. you'll learn how to take a person down from the standing position to the ground. you'll be able to counter punches and escape the wrath of a potentially stronger, more experienced and violent street fighter by taking away his main power on you...hand speed, punching power, distance, and aggression.
-takedown defense. in addition, you'll learn to properly defend against getting taken down.
-quick submissions from all angles. people who claim you'll be rolling around forever are wrong. first off, many submissions are applied from being ON TOP of your opponent...knee on stomach, side mount, mount, etc. you can disengage even the most tenuous opponent in less than a few seconds with a properly mounted choke, key lock, or arm bar, all of which can be applied from ALL angles.
-escapes. one user mentioned he didn't want to roll around with a bigger guy. okay fine. all the better to know how to escape from the ground when he's on you! not only will you become a strong ground fighter, you'll also be able to direct te fight...meaning you won't have to be STUCK on the ground if you don't want to be.
All in all bjj is invaluable to a fighter's repertoire. Combine the basic bjj submissions with practiced takedowns of judo and wrestling and you have a strong fighter.