I've been using Akka a bit but never tried FSM. Today I read section 9.3 which puzzles me a little.
As to "onTransition", I'm not sure it's the purpose but I find it may be useful in cases that the state machine is dealt with from transitions' perspective. But I'm not sure, "onTransition" an "when", which one is the better place to write corresponding actions.
In addtion, IMHO, "transform" and "using" don't seem be necessary as, I assume, everything can be boiled down to "when" and it probably will be clearer. I would like to know what you think.