I've been browsing for two years and this is my first post. I worked at BMW for two years and can shed some light about the quality between the different manufacturing plants. After seeing this topic come up countless times, hopefully this post will clear things up. For BMW specifically, the quality of the final product when it reaches the customer's hands should all be pretty much equal, regardless of the plant in which the car was produced.
That being said however, the manufacturing process from plant to plant is slightly different. The German plants--Munich, Regensburg, and Leipzig--adopt the latest advances in manufacturing first. These technologies then filter down to the South African plant. One example is the painting process. The German plants have a more advanced paint and paint curing process that has probably just been adopted by South Africa. In addition, due to the nature of the workforce, German plants tend to produce cars with fewer errors than the South African plant. This means that the South African vehicles require more rework when the finished car goes through final inspection--this was also reported by a leading European newspaper about one year ago, I forgot the source. After the rework, all vehicles should be 99% perfect. Any imperfections are not discernable by the customer.
There have also been years however, where the South African plant has produced more error-free cars than the German ones. Many people have mentioned that the SA plant won the award for highest manufacturing quality of European automobiles a few years back--when the e46 was being produced. For the e90 though, the German plants have traditionally been better. I remember one time a coworker was driving an SA-built e90 as an employee car. There were reported sealing problems in the interior that allowed water to come through. Other employees that drove SA-built e90s also reporter other minor problems along those lines. From a personal standpoint, I've heard less issues with German assembled e90s.
In regards to the German plants, assembly quality also varies. Leipzig is the newest plant, finsihed in time for the start of production of the e90 in 2005. My guess is that it would take some time to match the error-free rate of the older Munich and Regensburg plants. Those two plants also have been consistent winners in J.D. Power's annual studies. Regensburg consistently ranks among the tops in manufacturing quality for all European automobiles. They produce all 3 series coupes and cabriolets, the M3, and some 328i and the 335i. Munich produces all the X drive 3 series along with the wagon and the rwd 328i and 335i. Leipzig produces 328 and 335i, along with our 1 series coupe. SA for many years produced abouy 25% of the US mkt 328i. As of February 2008 however, some 335i for the US have started to come from SA.
To tell which plant in which your vehicle was built, look at the 11th character of your VIN. The following are the letters for each respective plant:
A, F, K: Munich
N: South Africa
Hopefully this helps everyone!