Judging from the two boards I frequent a lot, avoid a 2004. That year seems to have had a unproportionately large number of complaints. Granted this is all anecdotal evidence from people who decide to post on an internet board... meaning it may be an aberration compared to the real world.
Still, I dunno if you could say BMW had any recent "lapse" in quality. Mercedes had that prob IMO, but I dunno if you can say BMW did.