Via reddit.com, "The Vietnam of Computer Science":
"Although it may seem trite to say it, Object/Relational Mapping is the Vietnam of Computer Science. It represents a quagmire which starts well, gets more complicated as time passes, and before long entraps its users in a commitment that has no clear demarcation point, no clear win conditions, and no clear exit strategy."
A nice albeit verbose article on why using Object/Relational Mapping to store objects in relational database systems sounds like a good idea at first but then becomes a quagmire once you start trying to resolve more and more of the inherent differences between the two approaches.