Today I had to use the UserAgent extension to tell a site that I was using IE6 when in fact I was using Mozilla. The site came up, then it redirected to a message telling me that they had imporved the site, and to take advantage of it I'd need a more modern browser. Provided links to the download pages for IE and Netscape.
I offered a different UserAgent string, and compared the site side-by-side with Mozilla/Linux and IE6/WXP. I didn't see any differences. Had the pediatrician not already called the prescription in to that pharmacy, they would have lost a sale.