I was about to sit down and do it, but I'd like to know whether it makes sense to sit down and write a FSM from scratch, but one that fully utilizes Unity's strengths, for instance the drag & drop /declarative building of components. The way I see it, every state will extend MonoBehavior, to which one would be able to attach a set of other behaviors and tweak their properties using the Unity IDE, Separately, another behavior, the so called state machine will be responsible for switching between the states whenever needed.