Pro1 - You'll never get bored: You'll have all the awesome conversations together. There will be tonnes of giggling and beer chugging.
Con1 - He'll not treat you like a lady. You are his buddy. Yes there will be sudden romantic spurs but that's it. He won't get all chivalrous for you. He won't do it because you know it's fake and you will most probably laugh when he does it anyway!
Pro2 - No ugly surprises! You know what he likes, and he knows what you like. There's no sudden shockers. You know he doesn't like chinese and he knows you don't like Italian. So you won't have a fresh bout of disagreement over it.
Con2 - You'll take each other for granted a lot. He's always been there for you and you've always been there for him. So what's the big deal? The deal is you have expectations from a relationship which don't get fulfilled because of this.
Pro3 - You will never have to hide anything from the other. You can tell him all your problems and he can tell you all his. You both don't need to worry about either one of you judging the other.
Con3 - Sex will be awkward initially. That's because it's just weird to be naked before someone you have known for so long! Who knows all your secrets and everything about your past. It's just weird. But it will definitely wear off eventually.
Pro4 - You won't have to juggle between friends and boyfriend. If you guys are best of buddies chances are your friend circle is also the same. So everyone gets together in the same place. So you don't have to worry about your friends getting neglected.
Con4 - Your friends will most probably eat up all your alone time. They will hardly realize that you guys are now a couple and need alone time. They will keep dropping by and will most probably take offence if you ask them not to!
Pro and Con5 - if it's a long tern relationship you have been then a break up just won't be able to happen. Even if you want it to. There is no possible closure you can give to a relationship which is two things at once.