There's one very important thing when comparing Microsoft's ads to the Get A Mac ones.
Apple is attacking PC's. PC's are not Microsoft. While PC is standing as Personal Computer, in real life (and in Apple's ads) when we talk about PCs we mean everything that is not a Mac.
That's the important thing right there: Microsoft does NOT own any PC. Microsoft is a software company, that builds software for PCs. So, when the Get A Mac ads talk about PCs, they don't talk about Microsoft, but for many companies, without naming one.
Microsoft on the other hand, launched a full frontal assault on the Mac. And that's why Apple has the right to be pissed off.
However, I do find some right on Microsoft's part since some Get A Mac ads attacked Vista directly.
However, apart from all this story, I believe that Microsoft's ads are directed to people irrelevant with the PC world. The fact that Apple's products are more expensive is well known for decades now (literally). While Apple's ads target known PC vulnerabilities (viruses, incompatibility, major issues, hardware of lower quality), the ONLY thing Microsoft had to say is that Macs are more expensive? Wow. I think that they just dug their own graves with their ads.
Imagine FIAT to make similar ads about BMW's price to cars. It's essentially the same thing: Pointless.
One thing that really bugs me though, is Microsofts hidden insults and categorization behing those ads. I remember an ad displaying a woman saying ironically "I'm not cool enough to get a Mac". So what does that mean? That everyone who buys a mac is a show off? I am very far from this point, but I was really angry seeing the woman saying that. I remembered some years ago when people looked at me like I was some kind of alien when I was telling them that I have a Mac.
Anyways... we'll see how it goes.