I haven't really had a lot of ex's. But 'my first love' broke my heart.
We was together 2 years, from the age of 16 to 18. He cheated on me and
I was heartbroken. He said he wanted to stay friends and I agreed
because I didn't want to let him go and thought he would come back to
me. But you guessed it, he even did. So I spent about a year not dating
anyone else, waiting for him. I realised this was not good for me.
He is now totally out of my life, I am engaged to my lovely man and have never been happier.
So in my case, no, it's not a good idea to stay friends with an ex.