Might be useful in an atmospheric environment where lift is a factor, but lift would only realy counteract gavity if you're moving laterally at said critical speed, in relation to the gravitational source. If you're going straight down, gravity will accelerate you no matter what speed you're flying at, and it will decelerate you (or reduce your acceleration) when you're flying away from it. Unless you can tell it to only generate gravity when the dorsal or ventral surface of a craft is not pointing in the general direction of the gravity source, it will seem rather out of place.
Later!