If you're going to judge something, you should know it, not just know of it.
I would love to agree with you, as I usually do, but in this case I can't, at least not in 100%.
When I walk down the street and see a log of dog poo, I don't exactly have to kneel down, smell it, have a little taste and compare it to other delicacies to be 100% certain that it is indeed dog poo - I can see that it's quite bad.
Now's the time for my opinion:
The Call of Duty series are admittedly repetitive in nature. Unfortunatelly, unlike in the case of Halo, this is very hard if not impossible to avoid because they're based on the same engine AND they're all war games. They revolve around war. They have guns. That's the extent of repetitiveness.
The developers add new campaigns, tackle different conflicts and add different weapon sets - it's hard to accuse them of not trying out new things, they are indeed trying some.
If someone wants to argue that the formula has not changed one bit and it became stale then so be it - I usually say that the formula of Pokemon hasn't changed since the day the first game was released and yet people still cry out an unstoppable stream of praise for the games for reasons to me completely unknown.
The CoD formula hasn't been changed much because *it's a winning formula*. Video games are very much a buisness, it's pointless to change something that sells better than cure for cancer just because a few gamers nag. At the end of the day, the games are solid, especially when you consider the multiplayer aspects of the game.
UNFORTUNATELLY...
CoD games have one aspect in-common that spoils pretty much all the fun connected with them. This aspect is the CoD fanbase, which very much like in the case of Halo consists mostly of underage t*ats screaming into the microphone utterances unmannerly in nature. These gamers are not exactly concerned about the tactical values of matches or the general goal of each map - what counts is the final score, who showed who his 1337'ness etc. etc. while being obnoxious throughout the process.
Due to this specific group, which seems to be the great majority of the players using CoD servers, STUPID add-ons with STUPID overpowered perks are added on and on again, so that the 1337 kids feel more 1337 and buy the next installment. That is the cancer of the series, and one that will not be removed. Why? Because the game will sell worse. Would it make the game any better? Not sure. Some perks are actually fun to use.
Some people like grapefruits, others like oranges. At the end of the day, you have to approach the game and spend some time with it to judge. Nobody's telling you to finish it from cover to cover (i.e "have a taste of the poop"), just look at it objectively and make a judgement.