Apple's TV ads criticize Microsoft. I think "never" is too strong, but you should definitely be careful, and do it in good humor.
It's a crafty technique put to good use by Apple's marketing department. Whether it's something a small startup could use to put a dent in a market goliath, I'm not so sure.
By the way, I'm a PC guy.
The whole "I'm a PC" response from MS was decent. Unfortunately, they didn't go after the fashion statement base, which are the only Mac users who bother me. They're the ones that could be surfing the same Internet on a PC for 1/2 the cost.