I found this on WardsWiki, it's an XP Mailing List post by Kent Beck, that gives a pretty good insight into where the ideal of code reuse meets reality:

Although Money seems to be a perfect candidate for the Reuse Grail, it does not work out that way. Therefore, I carry around in my head a pattern language for creating Money and I reimplement it every time from scratch. It takes me about a day, integrated over a month, and the result is exactly what my clients need, and not a penny more.