Using (explicit) state machines in Rails is so easy, thanks to the gem of the same name(1), that we use them very often, with good results. I think that ease of setting them up and integrating them in whatever framework/language you use is a very important factor in their adoption.

1 current version: https://github.com/pluginaweek/state_machine

1a original unmaintained version: https://github.com/pluginaweek/state_machine