Glad you're enjoying the library, loads more to add yet! As for FlxVelocity it's not really meant to replace a tweening library such as greensock, it's really more to help move sprites around via velocity (rather than modifying x/y values directly, which stops them from colliding properly, etc).
I guess the end result may overlap a little with what the likes of TweenMax can do (having said that, I've never tried tweening flixel velocity values!), but if you don't need the full weight of a tween lib sitting around then it made sense (to me at least!) to keep it light and flixel specific.