i spar with a hapkido guy in my krav class. i'd say the one thing that hapkido emphasises where krav lacks is manipulating your opponent's balance. where krav tends to demonstrate techniques to quickly KO, HKD focuses on points of reference where the body is most susceptible to being imbalanced (the triangle). the joint locks and breaks are impressive too, but i feel that executing many of them leaves your guard down for too long - e.g. a wrist break requiring two hands leaves a window for the opponent to utilize their other arm or knees, kicks whatever. this is also evident in some HKD blocking techniques that require much more precision than the simple redirection principles of krav. i agree though, that they would complement eachother well.