Maintaining a "Relationship" with Your Ex: Is it Okay?

Is it really possible to keep a cordial relationship with your ex-girlfriend or ex-boyfriend, ex-wife or ex-husband? Does "ex" means past, something or somebody who is done and over with? Is wanting to be friends now a subconscious excuse to be around this person so you can find closure on unresolved things? Is wanting to be friends now, really a product of guilt you have for how you were as a partner? Do you feel you owe your ex, at the very least, your friendship?