People identify with the products they buy, whether it's a car, a computer, a phone, running shoes, etc.
Have you ever had an argument with someone over their favorite brand? Telling someone they bought the "wrong" brand is basically like telling them they're stupid. Not the best way to endear yourself to them.
SanDisk tried this in 2006 with their anti-iPod campaign "iDon't" that portrayed iPod owners as braying jackasses: http://www.ilounge.com/index.php/news/comments/sandisk-launc...
Sprint did something similar with the Palm Pre in 2009: http://jaysonelliot.com/blog/2009/06/27/the-wrong-way-to-sel...
People want to think of themselves as smart, savvy consumers. When they buy a big-ticket item, they spend time with the advertising AFTER they've already bought it, as a way of reaffirming their choice. When I worked on the Land Rover website from 2008-2010, we found that a significant amount of traffic was from recent buyers. They were coming to the site to feel good about the money they'd just spent.
Negative ads only work in political campaigns, where the goal is to suppress your competitor's turnout.
If you want to beat the competition, focus on your own positives. You can compare yourself to the competitor, but never tell their customers that they're stupid. You might get sales from people that already dislike Apple, but you'll never make a convert with an insult.