I have a tenant who is deathly afraid of mice. I have had a rather
intractable problem in this one building where she resides. I bait,
kill them off, and they come back a few months later.

I am in the process of filling all the interior walls with cellulose to
deprive them of nesting areas and to seal all the holes that they come
through. This is a very old building that shares common walls, so there
are lots of penetration that allow the mice to move around at will.
Unfortunately, the holes, plumbing penetrations, etc are all buried
behind the walls. Closing them up directly is not an option.

I need SOMETHING as a temporary stopgap measure, until I can finish
retrofitting the place. I have read that mice eventually build up a
resistance to those ultrasonic emitters. Has anyone here had good / bad
experience using them?