According to George Wiseman, there are 3 basic types of water injection:
1) Water Vapor
I've not tried the steam injection yet, but between the liquid and vapor injection, the liquid does wonders in quenching detonation and making power, but seems to reduce fuel economy. My guess is that the latent heat of evaporation effect cools the intake charge making it harder for the fuel to vaporize.
The water vapor injection, where you just bubble air (PCV gasses?) up through a water bubbler did get a tad bit more mileage on the vehicles I played with. Here you get the water injection benefits without cooling the intake charge substantially.