Gm has been running DOD/AFM engines in the trailblazers and envoys since 2003. As far a long term effect there has been non they simply shut off the fuel supply to those cylinders. I work for a chevy store in Metro Detroit and have a # of customers with the 5.3L AFM motors @ 120,000 miles plus and they are still running strong.
All DOD or AFM (same thing just depends on the year) Does is deactivate four of your eight cylinders when 100hp or less is required to propel the vehicle. In most cases even at highway speeds the car(impala) or truck(5.3L silverado's,suburbans,tahoes, and avalanche's) switches back and forth farely frequently do to road conditions(MICHIGAN, I'm not speaking for the flat parts of the country)such as up small hills your typically running all 8 cyl. and down hills cruising on just 4. In the larger vehicles simply driving into a 10 or 15 mph headwind can be enough to keep the cylinders from shutting off.
At city speeds @ or under 45mph the car will only deactivate the cylnder with steady driving speed (such as cruise control or a steady foot) or coastoing to a stop. At idle and under acceleration the engine will always run on all 8 (or 6) cylinders.
I have had two trailblazers with AFM/DOD and currently have a suburban. The system works great and typically gives me about the same fuel economy as my trailblazer customers with their I6 (4.2L) motors.
Please let all that I own be Red and Black and fast as F@$K!!