Taco Bell and McDonald's last week had an epic exchange after the former debuted its "Ronald McDonald" TV spot. And Ford took on Cadillac in what has been a late March defined by brand marketers having a little fun at another's expense. It raises the question: Do public rivalries make for good brand marketing? We asked a few folks at the 5th I-Com Global Summit, which kicked off proceedings this afternoon in Sevilla, Spain. Here's how they answered: Tania Yuki, CEO of Shareablee: Stuart Wilkinson, comScore's head of industry relations: Lucas Olmedo, CEO of online recommendations engine Fligoo: