It went stale for me with Black Ops. Modern Warfare 2 wasn't as good as the first but it was still kind of fun. Black Ops felt like the same thing a third time in 4 years and that turned me right off. I still have not bought Modern Warfare 3 and won't buy Black Ops 2, not until they are $15 or so anyway, which will take forever.
There was some entertainment to be found in the super linear cinematic stuff for a game or two but it wore thin quickly.