I agree and disagree with posters above.

I agree that many of the benefits of the diesel are intangible. The torque is great, the driving is great. Driving feel is obviously subjective. In my opinion, the diesel drives better than the 6 cylinder gasoline engine; the diesel, to my calibrated butt, doesn't feel like it has to work as hard as the gasoline engine to accelerate or drive. In 99% of my real world driving, I suspect that the diesel performs better than the gasoline engine would. It is rare that I need 100% of engine output...and to my taste, the diesel does a better job of moving my several tons of X5 around seem effortless.

I also agree that on a car costing as much as an X5, affording the gas shouldn't be a concern.

That said, I do greatly value the diesel's fuel economy. I have some years where I drive 30k miles or more. I can account for the car's purchase price (and to get a car that I'm happy driving, I have to spend some money). But if there are two cars, and one can do the job for substantially less $$ in operating costs (maintenance, fuel, etc.), then all else being equal, I'll take the lower cost option. I also personally feel that there is a substantial environmental benefit to better fuel economy. Sure, if I wanted the best possible results, I'd be driving a Prius or something...so it isn't the decisive factor for me, but it is a factor. And with fuel prices being as volatile as they are, given a choice between better mileage and worse mileage, I'll take better mileage every day of the week and twice on Sunday.

I guess I don't perceive any detriment to the diesel (versus the gas) from a subjective perspective...I see benefit. And from an objective perspective, the slight performance difference (which rarely, if ever, comes up in real world driving) is outweighed by the mileage difference--again in my opinion.

Regarding real world mileage, perhaps my experience is atypical. I've now put just over 4,000 miles on my X5 that I've been tracking mileage for (after an initial acclimation period that I'm excluding). All of that has been winter driving, on a new set of snow tires, with cold weather, winter fuel, etc. My driving during that time has been about 30% tollway (average speed between 60-70mph, depending on traffic; 4-8 lane tollways); 5% urban (stop and go), 30% rural (average speed 60ish; 2 lane roads), and the balance suburban (30-50mph). In that time, the car calculates my average economy at 27.3mpg. I calculate my fuel economy, hand cal'cd based on actual fuel usage and miles travelled, at 27.44mpg. No smoke and mirrors. No 'egg on the gas pedal' driving. I think that's substantially better than what I'd have in a gasoline model...and I think a substantial part of it is the driving that I do (a lot of 'solid-state' cruising, without acceleration/deceleration). I also suspect, based on past vehicles, that the mileage will go up this summer. So figuring fuel usage at 22mpg combined doesn't make a lot of sense for me.

I'm sure the mileage police will come in and arrest me for indicating that my average mileage thus far is 27.44mpg. But here are the figures, from when I started keeping track (about 5-600 miles after I got the car, and zeroing out previous fuel):
4131 miles
150.5 total gallons burned
27.448505 mpg.

I usually buy diesel at a local truckstop, and the price has been hovering around $3.47 lately. Let's call that $3.50.

$3.50 x 150.5 gallons = $526.75 in fuel costs, for 4131 miles. That's about $.127 per mile in fuel costs.

Let's compare to a gasoline engine. We can be generous and say that you'll get 20mpg average.
4131 miles
20 mpg
206.55 gallons (37% more fuel).

We'll be generous on the fuel costs, too, and estimate $3.35/gallon (a little cheaper than current gas prices, and significantly cheaper than premium fuel). That's $3.35 x 206.55 = $691.94 in fuel costs, for 4131 miles. That's about $.167 per mile in fuel costs (about $.04 per mile more, or about a 31% increase in cost per mile). Every 100 miles you drive, the cost difference is about $4. Every 1,000 miles, the difference is $40. Every 10,000 miles, the difference is $400. Is that a huge difference? No...probably not enough to make a difference in my purchase decision. But I'll still take the extra $400. If you carry that out to a year where I drive 30k miles, I'm saving $1,200/year on fuel...which is basically like having one month without a car payment--if you're figuring total cost of ownership. (Of note, the difference becomes more substantial if you use the EPA figures of $2.63 for diesel and $2.82 for premium).