I used to get about 39 mpg town driving in my focus 1.8 tdci. Now I accelerate a lot more smoothly, anticipate traffic lights ahead and change up at about 2200rpm and get 43mpg. It hasn't affected the amount I drive, even non essential journeys.
I live about 5 miles from the city centre and the 10 mile return trip used to cost me under £1 in fuel. Every 1p increase in diesel increases the journey cost by 1p. Nothing at all, but it has increased by 1p nearly 40 times!
It has mine, though it depends on your circumstances to some degree i would imagine. My 2.5 ltr lump only does 22 - 24mpg if i drive what i call normally for me. Due to the amount i'm spending on fuel, i now drive very light footed and have certainly noticed the difference with fuel consumption. Strangely get less stressed poodling about.