Originally Posted by 370Z Purist

It keeps the fuel spread out nearly perfectly in the cylinder, encouraging more complete combustion, which increases efficiency in the sense of more power (since more fuel burns) and cleaner exhausts (since more fuel burns). This is versus mixing fuel into the air at the throttle body area (well, a bit more past it). Fuel can "clump" so there are concentrated pockets of air and fuel mixtures, making combustion uneven and inefficient.

In general, it is significantly easier (chemically / stoichiometrically) to burn perfectly vaporized fuel than if were in a liquid state. Gasoline aerosolizes in air though, so it's difficult to observe in real time.

Direct injection is damn near the best thing you can get in any engine. But as mentioned before, a bad fuel pump or atomizer / injector will ruin any possible horsepower gain.

A good DI system provides good gains across the band, but particularly in high RPM ranges, giving good top end power (obviously, fuel has less time to spread out, so a good injector can do this better than a premixed engine, like the sequential mixer we have in the VQ37VHR).