On the face of it, that sounds amazing. If it was as amazing as you're probably imagining it to be, then privacy advocates everywhere would be cheering wildly.
Don't put your party hat on just yet. Although this is a step in the right direction, it's a small one, and its modest size is both underwhelming and disappointing.
There are two limitations to what we outlined above:
First, you've only got ten minutes to delete your message. That's great if you send something in anger, and immediately regret it, because it gives you the opportunity to take it back and get a "do over."
Unfortunately, once the ten-minute clock runs out, the message is part of the permanent record, just like it is now. That also means that saved conversations will be just as vulnerable as they are now, should a hacker breach your Facebook account, or the account of the person you're talking to.
Second, you can only delete messages you send. So building on the example above, you send a message in anger, but if before you can delete it the person you sent it to responds, you can only delete the message you sent, not their reply. The reply to your message will remain in the conversation log forever, unless the sender decides to delete it within his/her own ten minute window.
One final thing to be aware of here is that even if you delete a given message, Facebook doesn't. It retains a log of the message for an unknown amount of time. Ostensibly this is to prevent bullies from sending nasty messages, deleting them and thus destroying all proof that they ever sent it in the first place. As you can imagine, that solution leaves something to be desired.
All that to say, "Unsend!" is coming. It's going to be useful in certain circumstances, but for most people, it will fall well short of greatness.