Thanks for all the comments. Honestly, for me I'd probably pass on a diesel. I drive a 95 M3 and an '11 M3 convertible. This is for my wife who finds the bells and whistles entertaining. Going from a regular gas 3.0 to a twin turbo diesel will definitely be a thrill from 0-60 and certainly a noticeable improvement. Currently, she does about 80%highway driving and averages about 22mpg. Diesel is about 10-20 cents cheaper around here so the savings should add up. I was also always told that a Diesel engine would outlast a gas engine. We typically like to keep cars a long time so she understands this will have to last her at least 10 years till the next car.
To add, we just got back from a European vacation and found that gas over there runs about $10/ gallon. It seemed most people drive a car with a tiny engine (I think I actually saw a bmw 516....a 1.6 liter 5 series?) or a diesel. This presidents mentioned before that we should be paying the same as Europeans for gas, and it appears that we're being styled more and more so like Europe so preparations for future high gas prices seems logical.

I forgot to add that her commute is about 90 miles round trip and we take her car for any significant road trips. So the car will get lots of miles racked up. The extra MPG attained would quickly add up. I think the extra cost for the diesel was only about $2500