Well, technically, OpenStep.
Coming from a jack-of-all-trades experience in all things computing, I'd like to throw my two cents in with the whole Microsoft advertising campaign. I think there's a little bit of "everyone is right, and everyone is wrong" involved here. Let's leave the more obvious things aside -- the malware, viruses, etc., simply because those are external forces. Let's talk about inherent traits of both the Mac and Windows approach to user experience.
I've used a huge variety of systems over my time... CP/M, Apple ][ DOS, MS-DOS, Windows/286, Windows 3.xx, OS/2 2.0 - OS/2 4.0, Linux/GNOME, Linux/KDE, Linux/Enlightenment, Linux with no X server installed... Solaris, Windows 95-98-Me, Windows NT-2000-XP-Vista-7, AIX, XENIX, OS/400, Commodore 64, AmigaOS... and the list goes on for a while yet. In other words, I have pretty good experience with a broad range of user environments, from personal computers to mainframes and back again. I have seen really brilliant stuff on very limited hardware (AmigaOS comes to mind here) and absolute bone-headed design (ie, pre-Ubuntu Linux distributions with millions of menu options to overwhelm even power-users).
What I find particularly useful about what Apple has accomplished with Mac OS X is expressed in a very basic metric: how much does the system allow me to accomplish without the computer being a hinderance to my work? In the case of OS X, it is the best I have used with that respect.
Compare and contrast the user experience of a Windows machine to a Mac. Plug in a USB "keychain" drive. On the Mac, the OS adds an icon for that drive on your desktop, and creates a link in the /Volumes directory. That's all. On the Windows machine, a "balloon" pops up, with a sound, confirming that it recognized the drive. Another pops up soon afterwards, stating what kind of device it is. Then it pops up yet another balloon with the proper name of the device. Finally a balloon pops up to let you know the device is ready.
While these two sequences of events might seem trivial to examine, they provide deep insight into the mindset of Apple and Microsoft as software companies. Apple has a "zen"-like approach of keeping distractions to a minimum, breaking the user from his work flow only when absolutely necessary. They enforce this with strict UI standards for developers and tools within XCode to bring about a more consistent, low-key interface. The net effect of all this, plus the insistence on high-quality displays and interface hardware, creates a system that connects on a very deep human level in a near-symbiotic relationship. It's akin to a craftsman who has his one favorite tool; don't dare to make him use another one. The tool and the craftsman have become one. The same goes for a great computer interface and the user.
Now, let's look at Windows. Microsoft designed it around the "am I doing it right, boss?" approach. The system feels the need to inform you every time it's doing some piece of work for you. The problem with Microsoft's approach is that it is extremely disruptive to keeping a continuous thought pattern going with your desired task accomplishments. Windows requires the user to attend to a message that may or may not be trivial, each one requiring the same amount of momentary breaks in concentration. Add a more lax standard for developers' interfaces, and the Windows environment becomes cranial clutter; each application requiring the brain to slightly re-tool itself.
Now what on earth does any of this have to do with the Microsoft ad campaign? Well, it's quite simple. Microsoft can (rightly) go on TV and state that you can buy a cheaper computer. What it cannot claim is that you will be more productive with that Windows-based system, because most likely you will find yourself working around the distractions inherent in Microsoft's software philosophy. That is why Microsoft is betting good money over bad with this ad campaign -- because it's very difficult to pin down, in "bumper sticker" language, just why the experience of working on a Mac is better. Macs are better machines because they allow you to tap your creative potential with fewer encumbrances. The problem is -- put that in an ad.