As for DI, turbo-charged gas
As for DI, turbo-charged gas engines, I own one. I paid only $495 more for an Ecoboost in an F150 versus the base engine, versus the $13,000 more it would have cost me for a little Duramax in a Colorado/Canyon versus that base truck and base engine. The DI/turbo-charged, gas engines do have some merit, but the promise of this technology to save fuel is way over stated and requires very careful driving to get any advantage whatsoever. I can make this assertion from first-hand experience. I'm averaging 23 mpg in a full-size truck but have rarely put my foot on the pedal with much pressure and rarely downshift to accelerate at a faster pace. The few times that I have put my foot on the gas, the mpg began to drop immediately, and so most folks with the F150 and the 2.7 Ecoboost are getting 17-20 mpg as indicated on fuelly.com. Pretty good, but still very truck like. But for my situation, for what I need a truck for, and the configuration and gearing that I've got, and knowing what I know about getting the most efficiency out of this type of engine, It's doing pretty good for me; especially for the price.
This technology does equalize things a lot from a performance standpoint. Although they don't quite come up to the level of peak torque for any given horsepower as a diesel engine, they are much closer on the low end of the RPM range (example Ford's 2.7 Ecoboost--375 peak lbs foot of torque @ 3000 RPM, versus Ram's 3.0 V6 Ecodiesel @ 420 peak lbs foot torque @ 2000 RPM), yet, unlike a diesel, they also have the advantage of naturally-aspired gas-power as they continue to maintain that torque up the RPM range, and so, except for the new Cummins 5.0 diesel which seems to maintain torque above 4000 RPM (some how), most diesels will lose torque up the RPM range; whereas the gas turbos do not. This means that the engines like the Ecoboost have power and torque on tap just about all the time, in any situation.
But herein lies the problem with the DI turbo gas-powered technologies. This does not solve the problem that a spark-ignition engine must maintain a fairly-rich by comparison air-to-fuel ratio, and so when one stomps on the accelerator to spool up the turbo to suck in air, the fuel comes in fast and furious as well (contrary to compression-ignition), and so when I need performance, just like with a naturally-aspired, gas engine (or even worse), I get horrible mpg.
The advantage to the DI turbo-charged gas engine for me is the enhanced low end torque under light loads, and the small premium in price; particularly the latter. My 2.7 Ecoboost in a 4200 lb F150 drives and performs just like a V8 for daily driving, because those added components give it a little more drive around torque, meaning less downshifting, a more pleasurable driving experience, and better mpg. But if I put my foot on the gas, any advantage that I might have had with regular driving goes away and may actually be worse than a V8. When driving around easily and conservatively, the metered fueling of the direct injection and the slight boost of the turbo gives me a "big engine" feel except for the lack of engine braking due to the small displacement, but if I need or want to access all the power available with this truck, it's going to suck fuel as bad as a 6.2-liter V8.