Sure, it is easy, but it is also tedious and I use it in nearly every project. It would cut a lot of development time down if there was a plugin for that.
Moreover, I'd also like to have more or less independent FSM cells (one for each AI entity). It is easy to design AI using state machine, however in current form it is very tedious. Then some state machines run in parallel, so they shouldn't interfere with each other. For example game and GUI states, if you only use one global for states, as soon as you change the state, say open a game menu, there is a risk of breaking the game context.
What I want is a FSM plugin that would allow me to create independent enclosed machines or one big huge tree of states, whatever suits the purpose. It would also make managing events easier - when your caveman is hungry, he goes to hunt, thus activate Hunt state machine which handles hunting procedure. As soon as it finishes, it returns to whatever the player was doing before hunting. Then there'd be interrupts, such as spotting a lovely cavewoman, then Romance state machine is invoked. Some interrupts would stop the current procedure altogether or just pause it and resume after the interrupt is completed.
That's a lot of code to do.