Brazilian Jiu-Jitsu (BJJ) is a widely growing sport/self-defense style here in the United States. BJJ was originally popularized here in the U.S. by the early Ultimate Fighting Championship (UFC) matches between Royce Gracie and others in the early nineties. The system of BJJ utilizes leverage and technique to give a smaller opponent the skills necessary to defeat someone larger and stronger. When BJJ skill levels are similar, size and strength absolutely do matter, as they matter in a real life violent encounter.
BJJ is an excellent base system for those interested in learning how to defend themselves. Legitimate schools will give students the skills necessary to defeat bigger and stronger opponents, and students will achieve a greater level of fitness, and overall confidence as a byproduct. New students should focus on the basics as a beginner. Emphasis should be on learning the various positions and how to get out of inferior positions and achieve advantageous positions. Learning how to move your body on the ground, while in uncomfortable positions is a must in order to be successful in any type of ground encounter. Basic submissions will follow, but leave the X-guard and spider guard techniques alone until you have a good fundamental BJJ base. Those techniques certainly have merit, and can be used in a real life violent encounter (especially once we introduce weapons into the mix), but too often I see beginners trying to skip the fundamentals and work on the fancy stuff they see in competition like Abu Dhabi (ADCC), Eddie Bravo Invitational (EBI) or Metamoris.
New BJJ practitioners should train both with and without the GI (traditional uniform worn when practicing BJJ). The GI offers a different perspective and allows for the use of different grips, many of which can be utilized in a real life violent encounter if the attacker is wearing a jacket or heavy shirt. Training without the GI is also important as you can not rely on the bad guy wearing clothing that will replicate a GI. Having an understanding of how grips change, and techniques may need to be modified based on the clothing the attacker has on is a very important part of training in any form of grappling.
As you progress through your BJJ training you should always place emphasis on achieving a superior position (relative to your goals at that moment). Though some would argue that guard is as much a strong offensive position as it is a defensive one, if you pull someone into your guard (you on your back with them in between your legs) in a nightclub environment it can become very problematic very quickly. While one on one guard might offer certain advantages, when you add additional people into the mix, staying on the ground on your back may no longer seem like a viable option. New students training in BJJ for self-defense should also always be thinking about striking while grappling. Most BJJ schools are not going to have students striking while they roll, however from a self-defense perspective, it is imperative to be thinking about when you can be hit and when you can hit the attacker.
As your progress through your BJJ training and become more comfortable with the various positions, transitions, and basic submission skills, you should start to incorporate the use and defense of weapons into your training. Training to deal with an opponent who may be armed is vital, and most BJJ schools do not include curriculum that addresses this particular area. There are many quality instructors who are specializing in dealing with this very area. I suggest you look into them, and get in a class, rather than leave a large gap in your protection skill set. BJJ is a great skill to have and in my opinion it is absolutely necessary for anyone serious about their self-defense skills. Seek out reputable, quality instruction and put your time in on the mats. There is no substitute for training and repetition. No shortcuts. Do the work and you will get the results you are looking for. See you on the mats!
-Adam