Previous thread where filtration was discussed with lots of good info:
http://www.bimmerfest.com/forums/sho...d.php?t=461450
And official specs for our d's fuel filter:
http://www.bmwtradeparts.co.uk/PDF/W...uelFilters.pdf
It's actually spec'd pretty good, and also separates water (although you can't drain the water, so if you get a big slug of water I'm guessing bad things would happen, as evident from the example pointed to in the first link).
To be fair, I've owned 6 diesel vehicles, all but the 335d had water separators, but I've never found more than a drop of water in any of them.