There's no doubt RUC hits the diesels in the pocket, but its still cheaper to run than my last two cars...
I do 650 kms a week and my 123D does it on around $75 including RUC (a full tank is around $50 and I get 1100 kms plus RUC of $58 per thou). My 328i cost $145 to fill and did 620kms (when gas was $2.20) and I briefly ran a Polo GTI which was $90 (@$2.20) to fill and did 670kms a tank.
Roughly, $ for $ including RUC I figure it makes the cost of diesel motoring about the same as petrol, but its the much better economy from a tank and the huge torque of the diesel which makes it fun to drive and cheap to run. Torque (295 lbs) as a E90 M3.
I was never a fan of diesels until I spent 3 months driving one in Europe - not huge power and not peaky, just s***loads of torque and handled really well, just fun to drive.