I had a Golf GTI 2.0 DSG petrol and used to get 28mpg if i drove it with care. Before that I had a 3.0 Z4 and got about the same mpg, I sold the Z4 because it kept going wrong! I do 70 miles a day and my fuel bills were huge. Ive now got a 330d,its faster than the golf was and very nearly a match for the Z4. The fun thing for me is being able to afford to drive it quickly rather than having to nurse it all the time. I get 44+mpg and get home a lot quicker.
But you only save money with diesels if you do medium to high miles as they cost more to buy,but they do hold there value better. 5p per ltr between petrol and diesel though makes no real difference