For me, the lifter failure was just sudden with no signs leading up to it. To this point, i've not known anyone who's kept their AFM enabled and NOT eventually had an issue at around 135k-150k on the vehicle. It will happen, just a matter of time.
My wife drove her 07 Tahoe LTZ from 125K up to about 154K with AFM still enabled. No issues with oil consumption, no bucking/hesitation etc. when switching modes, nothing.
Disabled AFM first with Range and then with i1000 and the thing was still going strong when we sold it a couple of weeks ago. But yes, disabling the AFM is one of the first things anybody should do when buying one of these things, regardless of the miles.
AFM in my wife's new 2010 will be disabled shortly.