Honey-child, i think you know the answer to that question. Would you respect a woman who takes back her ex after leaving his "now" girlfriend?
I am sure there is a good reason why he is your "ex". The question is, are you willing to go through that all over again and more importantly take on someone else's sloppy seconds?
Come on now, the world is full of men. There are honestly plenty of fishes in the sea for you to pick from, don't feel like you owe this guy anything or that you have to settle for just him - that is ridiculous.
Good luck =)