Im going to be really honest. This is from my own experience.
I dont think its a good idea to be friends - especially really good friends - with an ex. I thought I had a great friendship with my ex boyfriend. Things between us didnt end on a sour note, although there were some hurt feelings & emotions that festered for a little while after our break up.
When we began a friendship, everything was cool because neither of us wanted to be involved with each other romantically. I however still had expectations of him simply because of the previous relationship we had. Not expectations in a romantic way but expectations like "I need him to move this piece of furniture across the room for me now" expectations. Whenever he could not be there for me in the capacity that I wanted, I would get offended. I felt that way because i still had the mentality that since we had this relationship in the past, then it somehow put me on the friendship pedestal, even though technically I was in the friend category like everyone else. It was hard for me to be cool with not being a priority in his life like I was when we were together. I felt like no matter what, we had that special bond and I am forever to remain special, regardless of whatever else is going on in his life. That wasnt the right attitude to have and I had to check myself. I raised up & faded to black.
Many times, going from being in a romantic relationship to being just a friend is like a demotion. You really have to be honest with yourself to assess if its something for you. We still call each other to say happy birthday, merry christmas, etc but thats about as far as it needs to go IMO.