bobsr:
I just bought exactly the same thing; a 2012 Yukon Denali 6.2 with AFM and 99k miles. The system came on all of these vehicles from the factory.
First of all, while the concept is the same as the old 8-6-4 system from the early 1980's, the actual implementation is vastly better. Modern electronics and variable valvetrain geometry have supplanted the jury-rigged, primitive systems of old. When it works, it works very well, and isn't especially noticeable unless you have an aftermarket exhaust system, which audibly reveals V4 mode. GM decided to develop the system and install it on many vehicles to help them meet government CAFE requirements. Anecdotally, the savings is around 5% on average. Actual fuel savings varies with terrain and driving style.
Secondly, by 2012, it is my understanding that the system had become pretty reliable, provided proper oil change intervals were followed. The system relies on solenoid valves to divert oil pressure to retractable pins on the valve lifters, allowing them to collapse. Dirty oil reduces the likelihood that this system will work properly.
Thirdly, having said that, with unknown service history and the accumulation of miles, it stands to reason that AFM is nevertheless a potential failure point. For that reason, I chose to deactivate AFM on my Yukon shortly after I bought it. I did this using a custom tune. There is a plug-in device, called a Range module, that effectively does the same thing. After studying the operation of the AFM system, I determined that if it is never actuated, there is no risk of failure; at least none that I can determine. For me, the mileage benefit of AFM was negligible. And I wanted the peace of mind of knowing the valvetrain would last.
I would not, and did not let the existence of AFM deter me from buying what is otherwise an excellent vehicle with a fantastic engine.