I just had a trooper get mind controlled by a sectoid and successfully stunned him with a field medic using the arc thrower. It displayed as a "stun 2" and indeed the trooper didn't act on his following turn. In the turn right after that, however, the trooper was still mind controlled and happily knocked out my field medic with a frag. Managed to reproduce this a second time, too, from a save game. If a fix could be found that would be awesome. The idea for the ability is really cool, so it's a shame that it doesn't quite seem to work as advertised.