Customers are and should be the top priority in any line of work, but customers also need to know that there are borders beyond which they may not pass. It’s not OK to get abusive, it’s not OK to go nuclear and dump venom on a customer-facing representative just because your issue wasn’t addressed Right Friggin’ Now!
Furthermore, if you as a customer cross those lines then you deserve to have that rep’s manager, in a polite and professional manner, put you firmly back in perspective.
Josh Ledgard has a very thoughtful post on just this topic as relating to Microsoft: scooblog by josh ledgard : Anyone but you again. Be sure to read the first comment, as it’s a great example of someone who’s over the line. What a hysterical affirmation of Josh’s point.