Most Helpful Guy
1.) You know you get along to a certain degree because you were friends before.
2.) If there has been any awkward sexual tension before between you two, it now has a place to go. (Sometimes people like each other and are just too afraid to make a move)
3.) Some people treat relationships like their best friend, so hey, you already got that part done.
1.) It may be difficult to break through the "friendship" mentality into the dating mentality.
2.) If the romantic relationship is ruined, your friendship may be screwed over too, or at least become very awkward.
3.) You'll learn everything about your best friend, and he may not be as pleasant as you first imagined.
Most Helpful Girl
Cons would be mostly at the beginning, when you're sure you look at them as more than a friend and have to decide whether or not you're willing to risk losing your friendship over letting them know how you feel. If it works out and you guys start dating, it's a bit difficult to make that transition to best friends to couple, so joking around about it helps a lot.
Pros? You start the relationship a few steps ahead. You already know where both of you stand on many issues, you might have already met their family so there'll be no fancy introduction. Your emotional connection is already pretty sound too, so once you guys are both ready, it's just a matter of your sexual relationship catching up to the rest. :P