there is a number of factors you have to look at: 1) how many miles per year you drive 2) the cost of petrol vs diesel 3) purchase price 4) residual value for petrol/diesel car 5) how fast you like to drive (IMO).
So yes, diesels are more expensive to buy, but they keep much better as second hand car. Also considering you drive like 15,000-20,000 miles a year and with the diesel being more expensive, it can take 3-4 years before you get the value back in fuel savings..
Over here, diesel has been more expensive than petrol for over a year now, because most of new cars are sold with diesel engines anyway.
But the appeal of diesel is obvious. Take a regular BMW 320d for instance, it's got 177 HP, I'd say it is surprisingly quick and nice to drive, and does 57 MPG, so even if you do a 100 MPH on the motorway it doesn't do less than some 45 MPG.