One real world example are those registration links you get via mail to confirm your account. It happens very often to me that I have to re-request the mail, to get it into my mailserver.
Oh, you'll get those in an hour or so, when their mailer retires. I have never had a problem.
I did kill greylisting, though, because it breaks when the mail server that first tried to send the message is not the one that tries to send it again. Gmail does this, for example, and unless you keep up to date on Google's internal architecture, you are going to lose mail from Gmail users. Combined with being tired of waiting an hour or more for every email, this killed greylisting for me. Now I let the spammers deal with spamassassin, which is quite nice if you bump up the scores on the content and URI blacklists.