I bought and enjoyed COD4 when it came out.
Never thought about buying MW2.
Got blackops because I thought it looked neat.
Never thought once about buying MW3.
Not sure why. I ALWAYS play through the single player of these games before I decide to buy them. Blackops I really liked. All the other game were really mediocre. As for online play, I never really took it seriously. If I want competitive FPS I just load up come CS1.6.