i have a 2004 320d for last three years, 143k on the clock and still driving as well as the day i got it. i keep it serviced regulary, cost of service is no dearer that any other car ive owned. 45mpg, 500 miles on a full thank. i have nothing but good things to say about the car, apart from service only thing that needed replacing were rear shocks.
early 2004 and earlier cars the turbo going was a common fault, an expensive fix.
i once drove a 407, nice car to drive but interior had worn a lot on the one year old car i drove. next door neighbour has one and raves about it.