Yes I agree with Rico that a side kick to the knee cap is very very dangerous because the damage has a crippling effect. Although its not an easy target with lots of hours drilling and practicing this technique it can be done. This is one of my main techniques I emplow if facing a large aggressive adversary even then you would have to be real careful if employing this technique because you could actually get sued and have a law suit on your hands even if you were just defending yourself.

Thanks for the replies...I'll keep this in mind. Of course I know its done with the punches, I just needed "them" to come to my level so I can finish the fight quickly...I don't like the idea of continuing the fight and then someone else joins in.
Does it matter if the kick is towards the inner side or the opposite side of the opponent's knee? If its one or the other, which is more devastating?

Firstly, you should aim for the knee with the weight on it, for someone who doesnt do MAs that would be the front knee nearly all of the time, which is very convenient. The more weight on the knee the easier it will be to break and the harder it will be to defend, since it would take longer for the opponent to shift his weight.
If he knows wing chun and has the weight on the rear knee then i would not advise you to go for the rear leg because it would leave you open for attack since you will be offbalance too near to the opponent for comfort, so what i would do is go for the knee of the front leg which wont break but will hurt, and then slide down the shin and land on his foot trapping it and then go in with hand attacks.
If you go for the inside of the knee then it should result in a break if done properly, if you go for the outside it may result in a break but it may also just make the opponent twist down onto the floor, in which case your foot should keep contact with the back of the knee and then jump and land on the exposed ankle with your other foot, breaking that instead.
I m not sure if what i am saying makes sense.

just be careful of trapping/breaking your own hands in an attack. Punching certain areas of the chest etc can result in a "reflex headbutt" from the opponent, or your hand getting trapped. A lot of people will have a reflex to pull their elbow back when hit (especially around their ribs), so make sure your fist isn't striking anywhere in that region, or it will get broken/trapped. Just a few things worth considering that don't necessarily manifest themselves in sparring, particularly if all your sparring partners have conditioned themselves not to perform these "reflexes"