My best friend, is my ex..

Is it a crime? (that came from Sade's song that happened to be playing) So, I've had this conversation with numerous people, whenever I speak about my ex I always include "oh which happens to be my best friend." Here's the back story, I met her when I was 18 and we dated from my age of 19-23 and still through the good and the bad we have remained close.
 My question, Is this problematic? When I first think of the answer I say yes, if you're entering into a new relationship how would you feel if your partner was indulging pieces of them they should be sharing with you with their ex? Mmmm tricky! The other part of me feels as though this is a person that knows me inside and out, (no pun intended) the good and the bad how do you just stop having them as a confident?
 There is this fine line, as I get older my understanding for things have shifted. I want to know what do you think? Can your ex remain your best friend or is it better to draw the line between you two a lot thicker?

Let me know your honest thoughts, I know I am not the only one wondering.