XP was released in 2001, It was really good for its time. Most people kept using it(beacuse of Issues with Vista) till 7 launch(2009).So for 2009 XP may seem old and outdated. But the fact that after failure of Vista XP was able to keep OS marketshare for Microsoft is astonishing.
I tend to disagree. In my opinion it is the worst OS ever created and the reason it's popular has nothing to do with quality.