i think it depends on a lot of factors. probably the most important being whether or not the break up was mutual. if it didn't get too serious and shit just got weird or you just weren't feeling it, it's usually not difficult to be friends. even if you're in a situation where the relationship just kinda runs its course, it was nice while it lasted, it's just old time to move on, even that can be ok depending on circumstances. but i've never been able to be friends when it's a bad break up, where the trust has been broken, it's hard, but mostly impossible to be friends. however, everyone is different. but in my personal experience it really doesn't work out and when it does you're not really "friends" in the websters dictionary sense of the word, but more like acquaintances in passing.