We have a 2008 MT M3 and a 335d. 15mpg in the M3, 28 in the 335d. Those are driving mix city/highway 50/50.
Half the time the 335d feels more powerful as the torque is always there. The M3 shines when you get the speed and the revs up, but it's so easy in the 335d you feel like you're having to work harder to do the same in the M3. Don't get me wrong, we love our M3, but it's going to get traded way before the D.
Oh - and there are some 335d (US spec) upgrades available now also
