Consumer Reports long-term reliability tests are determined by input from subscribers, not exactly a scientific poll. JD power results are from random samplings. So you tell me which one is more apt to be a fair assessment?
There's still self selection w/JD Power's "studies" since not everyone will bother sending the surveys back, just like w/Consumer Reports. I've found that the systems that CR points out as trouble spots on vehicles that my parents and I have owned are pretty accurate. With CR's studies, you're only supposed to mark whether you've had a major problem (in terms of cost, downtime, premature failure [I believe] or compromised safety) in a particular system. With JD's studies, people might consider "wind noise" or "fuel economy" to be a "problem". See http://www.usatoday.com/money/autos/2005-05-18-quality_x.htm. If you buy a Hummer H2 and it gets crap gas mileage, it counts as a "problem" on JD's IQS? Since you cited GM has having the "highest quality plants" in North America by the somewhat dubious metric, notice that outside North America, Toyota, Nissan, Jaguar, BMW and Porsche have plants that are equal to or better than the "best" GM plant? The other problem is that JD's so called "long term" studies (like at http://www.jdpower.com/news/releases/press...asp?ID=2005089) only are for 3 year old vehicles. How many people buy a car and dump it within 3 years? If it's sold, someone else is most likely going to own it and keep it beyond the 3 year mark. CR's stats cover 8 model years.
GM is fine for the first 3 years.... thats how long their warranty lasts!... But they have trained thier engineers to strategically build tolerances within narrow limits so as to purposely loose reliablity after that. You ever wonder why the radiator is "barely" big enough to do the job?... it works providing everything is working perfect, including the auxillary fan that helps, with its funky clutch system that also has to work flawlessly. They build alot of links into the chain to go wrong. The sad thing, is its not that GM "can't" build better, or that they don't know how, they don't want to. The way thier programming is, they don't believe its to their advantage to build rigs for longevity. They don't possess a spirit of excellence, only greed. They don't want to build something that works for 10+ years, they want to see you come in and have to buy another before that.