I love a quote, especially from a fictional character. 'How lucky I am to have something that makes saying goodbye so hard - Winnie the Pooh' is a really nice, not overly sentimental, way of saying hey, I'm going to miss you and what we have together.
 
Ideal as a card when for when someone is leaving work or moving away, you could even send it to someone you want to break up with. It sure beats some of the less pleasant ways I've been dumped...