Full disclosure: I've got 30,000 miles and 2 years on my Hyundai Elantra - I hope I am right. So far so good. Flawless.
Initial quality ratings, in my mind, are only "assembly quality" -- how well it was screwed together. It says little of the quality of the parts used, or the design. Cars like the Mercury Sable and the like traditionally do really well on IQS, simply because they have been made for so many years without change, and the assembly lines have been perfected and workers know how to do it well. But throw 3-5 years of use at those cars and long term ratings are average at best. Long term quality ratings show the quality of components used, and design.
That said, the Koreans have come a long way. They have improved a lot faster than the Japanese did.