To clarify, most hybrids do get excellent highway mileage, but that's because they tend to be designed for efficiency (smaller engines, lighter weight, more aerodynamic, etc.) in general. A Prius, running 100% on gas, is still a very efficient car. The same body, at the same weight, using the same engine, would get essentially the same highway mileage.

The real benefit of being a hybrid per se is in city driving, where you recapture some of the energy wasted in speed up/slow down driving.