Before I went back to uni in 2012 I was considering getting a new car - when I compared the diesel I wanted and the equivalent petrol (A5 3.0TDI vs A5 2.0TFSI - both DSG quattro variants, same sort of power and on-paper performance), I worked out that it would have taken 4 years of diesel ownership with my circumstances to see a saving when factoring in initial purchase price, fuel, servicing, tax, insurance etc. etc. Diesel isn't always the cheaper option.
As it stood I did go back to uni so I kept my A3 2.0TFSI. My wife has a A3 1.9TDI. Mine will do 440mi on a tank. Hers does around 600mi on a tank. So my car is roughly 25% less fuel efficient. But diesel costs maybe 10-15% more to buy. When comparing mine and my missus cars, I'd very gladly pay a few more pounds a month in fuel and have a car with twice the power that is more fun to drive. It is a no brainer for me.
Diesel isn't the be all and end all. In a lot of situiations they can be really great, absolutely no doubt about that. But people need to sit down and work out whether it is the best option for them. In a lot of cases it isn't.