I find my 2013 X3 build quality to be about the same as the 2012 3-series.. but I also noticed a drop in the quality in the 2012 when compared to the 2006 3-series. that thing was a tank.
the seriously subjective things I use to judge build quality;
- interior matierials, dash, panels, buttons.. how clicky are they? do they feel cheap and hollow or does it give a nice solid click.
- exterior materials, paint and trim.. how does the door feel upon opening.. does it have a good weight to it, or does it fly open and closed. does it give a nice solid 'thud' when i shut it.
- noises; how high can i put the speakers/bass up before i start to hear distortion? when I'm making a long sharp turn, are there any 'creaks or squeaks' coming from anywhere in the car? wind noise on the highway?