Are vasectomy's reversible?

Me and my fiancee are thinking about him getting a vasectomy. He's a little reluctant because he wants children in the future, and is hoping one day I might change my mind. I know I won't but he's still worried about a reversal. So can vasectomy's be reversed? How long after is too long to get it reversed? What are the odds of him getting me pregnant after he's had it reversed?