From experience, i can say that it is not that great of an idea.
I liked it at first, having the seductive secret meetings.
Knowing that he wanted me sort of made me feel special.... but later i realized, that there are those times when i was kept around for him to call late at night after everyone else he wanted has rejected him... he knows he can always get it from me (we are buddies). . . . so doest that make her a "special friend", or a last resort that will put out?

needless to say, I stopped this destructive behavoir years ago, and i would not suggest it to anyone. It CAN ruin friendships, spark jealousy, and possibly ruin the chance of starting up new relationships with other people. it can be had to get rid of a "sex buddy" because they will say, "lets just keep it between us"

Dont fall into it.