I have pursued girls with boyfriends, and it never worked for me. Here are the problems/things that I came up with.
1) if the boyfriend lives in the same area, the girl's time is usually tied up in activities involving him, which limits you.
2) on the otherhand, if she is in a long distance relationship, this helps you out. Chicks in long distance relationships (at least in college) tend to not go out as much and associate mostly with large groups of female friends, from my experience. Thus, there is less competition.
3) often, the chick feels the need to let you know that she has a boyfriend. This might be because she doesn't want you or she doesn't want to feel guilty for cheating because you knew the consequences.
4) sometimes, when a chick tells you she has a boyfriend, she will assume that your subsequent advances are friendly only, and not sexual in nature, because obviously, she has a boyfriend already. Thus, you have to be less subtle to avoid that friend zone.
5) I am theorizing that faster seductions would be better in the case of trying to cheat. This gives the girl less time to think about the moral issues and negative consequences involved with cheating. On the flip side, you need enought time to build rapport, make her feel comfortable, and isolate her.
6) I don't know anyone who has stolen a girlfriend or boyfriend from someone and has been able to make the relationship last more than one night. I am not sure it really happens.
7) you have to weight the moral issues involved. Cheating and causing others to cheat is not for everyone. I didn't have moral qualms when I was trying to get with chicks who have boyfriends in long distance relationships, but I kinda do now. If you are shakely on the issues, play it safe and don't cheat because I know people who cheated and they regret it immensely.
Peace.