I can't say for sure they're bad en masse, but they did have to send an engineering team out from Bavaria to look at some issues. The oil pump they used in the 2006 models (or so I have read -- not sure if this still applies) is an electric design, which pumps only the amount of oil that is needed, and not full-out all of the time. As a result, there is some oil drainback in the head, resulting in lifter ticking, which gets a little rough on the cams. Automobile magazine had this problem on their test 330I, and they got a new cylinder head out of it, IIRC.
I haven't read much else that I can recall, but I did drive a 325I of the same vintage and really liked it until reading about the cylinder head.