I was told they won't get rid of mice that are already there but will stop any more getting in, and that seems to be what happened with ours - we caught four after we'd plugged the things in but it's about 8 months since our last one now.
btw - if you do get one, don't look after your DD's friend's rats for the weekend & forget to turn it off - I wondered why the poor things were so jumpy.....