Personally for me, I am really trying to avoid any negative review at all. My feeling is that my site is not as big as the larger players in the industry. People will google my site name to look at reviews before they buy. One negative review can show up in the SERPs and that can affect business, so even if they are upset or rude, I try my best to listen to their concerns and offer something like a free gift or at least show I am willing to work with them. Of course there is that tiny percentage of people who are just looney and need to be told to shop elsewhere.