I took the risk and ended up marrying my best friend! Admittedly we only knew each other for three months but it was an incredibly intense friendship that developed very quickly. It'll be nearly 19 years this year and he is and always will be my best friend.
I think pros and cons already covered by others so won't duplicate.
I have a good friend who I've known for years and would never consider dating him because he falls under the "brother" category.
I think if you've known your best friend for years then you've both had time to figure out if you "like" them more than as friend and also to check if you're compatible ie physical/emotional/intellectual and spiritually (in no particular order). If they tick the boxes then I would definitely take the plunge and risk it all.