Well, 60% of the people who buy it are tacticool 12-year olds, 35% are the people who stake their social status on their K/D ratio, and the remaining 5% are the ones who buy it to make fun of those people.
I have not played any COD game since the second one, so I can't judge the new ones.
But one thing I can say for certain about each COD's campaign (and from what I've scene on the newer titles) is that they pack it with lots of action and adrenaline. And I love that.
Before COD 1, there has not been any FPS that brought as much packed action and epic battles on a large scale.
I don't like playing games online other than Left 4 Dead, so when I'll eventually buy the newer COD titles, it'll be for their campaigns. (It's also why I'm waiting until the price drops a lot before buying.

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http://www.youtube.com/watch?v=ijolI14sXEM pretty much established how to make an arcade war-related FPS for the future games to come. It was a great addition to gaming history and pretty much beat Medal of Honor's way of doing it, which was becoming shallow since Allied Assault.
Call of Duty 1 has some of my favourite gaming moments, http://www.youtube.com/watch?v=a9D7p9jw4Vw being my favourite of course.