Taking a friendship to the next level

Picture this: you have been friends with someone for a long time and you slowly start to develop feelings for them.
What do you do? Do you take the plunge and start up a relationship or do you keep things the way they are?
If you do take the plunge what happens if it doesn't work out? Then not only have you lost a partner but also a friend. And what will that mean for your mutual friends?
But if you are already good friends then you already know that you get along so there is a good chance of you being able to make it as a couple.
Or you could keep things as they are and stay as friends but then there is always going to be your underlying feelings.
I think this could be one of the most difficult decisions to make.
Have you ever been faced with this dilemma? What would you do?