Europe went Diesel-crazy in the late 90's. Four cylinder, turbocharged, two litre engines became the standard choice for most of us. Those engines produced around a hundred horsepower, but they felt much, much more powerful. We "discovered" torque.
Torque is what makes you feel an engine is powerful, because it's what you can find in the middle of the rpm range. Those early Diesel engines had lots of it, specially compared with the naturally aspired 1.6 most people were used to. Like twice as much.
Many people still buy Diesels today because they think they will last forever, consume no fuel whatsoever while delivering all the torque one can dream of. The truth, however, is quite different.
Modern Diesels are über-complicated engines. This means there are a lot of bits that can go wrong at any given moment. So, while they can still work for ages, they will need to be fixed on the way there. Fuel economy hasn't improved all that much for the last ten years for Diesel engines, while there has been a huge improvement for petrol engines on the meantime. Finally, modern Diesels still earn the torque trophy, but modern turbocharged Ottos aren't as weak as they used to be.
All things considered, is it still worth it to spend a couple of grands more to get the Diesel engine? Well, the answer depends on the car you are looking for, but a few general rules can help.
Rule #1: Do you use your car on a daily basis? Like commuting to work, school runs, shopping, etc? If the answer is yes, go for the Diesel. If the answer is no, go to the next rule.
Rule #2: Are you planning to sell it within the next 2 to 8 years? If the answer is yes, go for the Diesel, resale value will more than double the extra money you paid in the first place. Answer is no, follow the next rule
Rule #3: Do you tow or use all of your car's cargo? If the answer is yes, again, go for the Diesel. You need that torque, and if you were to buy a turbocharged petrol engine, you must have in mind that fuel consumption skyrockets under heavy loads.
Rule #4: Are you young? This is the trickiest rule. If you are young (and by that, I mean you have yet to settle down) you are probably better off with a diesel. Once you get married, have kids, and so on, your life will be rather predictable. However, if you are young enough, you might see yourself dating some who lives 50 miles away, while your new job is 40 miles away... in the opposite direction. Chances are you'll change your significant other, your job, you'll move, etc. This means you'll need your car as a tool to match your life bits.