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Generally speaking, a electric assist motor bolted onto a gasoline (or diesel) engine improves power, but does very little for economy, if nothing else is changed. You can recover very small amounts of energy when decelerating, and perhaps you can upshift sooner because more torque is available at lower RPM.
The addition of auto-stop helps in city driving as well, but that's harder to implement because it requires some integration between the engine's computer and the motor's computer.
On the highway, the only added benefit you may get is that a 3 phase motor and DC-DC converter may be more efficient than an alternator at creating 12v.
Those hybrid systems that help significantly in city driving typically do so by having the system highly integrated into the transmission. For instance, Toyota's HSD can toggle the gasoline engine on and off on its own while driving at a steady speed, basically doing pulse and glide without causing any speed variation. It can also keep the engine off during very low speed crawling, only firing it up to top off the battery.
The major benefit of hybrids on the highway is that they tend to incorporate much taller gearing, and smaller engines, than would otherwise be acceptable to typical drivers.
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