Kung Fu is really a catch all term for Chinese martial arts, and as such refers to a multitude of styles; hundreds, perhaps even thousands. So to ask whether Kung Fu is effective for fighting is a huge oversimplification. Some styles are sport styles, and as such have a pedigree in sporting competition: Sanda, San Shou, and Shuai Jiao are the most common styles, but these terms also refer to rule sets under which stylists from more traditional styles compete. And many of these traditional styles have competed on Lei Tai (a raised platform that fighters from various styles of Kung Fu compete on). Hung Ga, Pak Mei and Baji are common examples. Other arts, like Pao Chui and Xingyiquan have a history of military and mercenary practise. There are many practitioners of various Kung Fu systems who like to claim that Chinese martial arts are the greatest form of close quarter combat available, but this is nonsense. In fact, the constant repression of martial arts at home by a slew of governments in China, mixed with the commercialism of some modern teachers, has damaged Kung Fu's reputation.

It is also true that Chinese martial arts have a long history of non-combative facets: there are performance arts: Chinese Opera, and modern Wushu both fall into this category.

Providing you have some potential and a realistic will to fight, there are 2 ingredients in any form of martial arts training that simply cannot be done without:

1 - Realistic techniques: if you want to do well in real fighting, your style must contain simple, effective, techniques that you can pull off under pressure and regularly succeed with. That's not to say that it can't include more unexpected maneouvers, but there must be a realistic core of basic techniques. Things like jabs, crosses, front kicks and throws. Sport arts in particular have realistic techniques. However, it is also important to make sure the techniques you are training equip you for the type of combat in which you are expecting to be engaged. Training boxing alone will ill-equip you for MMA. Training MMA will be of limited use if attacked with a blade. And, as the old saying goes, never take a knife to a gun fight (sorry, Escrimadors).

2 - Realistic training: the best techniques in the world aren't going to help you unless you practise them on other fighters of equal and greater skill than yourself in a completely non-compliant way. For this reason, it is important to engage in full contact sparring if you want to ensure that your martial art is effective.

Of course, there are other important factors that contribute to fighting skill -- various drills, different types of conditioning, a study of psychology, strategy, and tactics -- but it's teh above two points that most traditional martial artists fall down on. If you include them in your training, then your martial art (Kung Fu or otherwise) will likely be formidable.