Product review (5 stars): Works great for Wifi Thermostats
Used a few of these for adding Wifi Thermostats (Radio Thermostat) to my home.  Older homes do not typically have the common wire needed for powering the newer thermostats.  These transformers are not small but I was able to hide it in the wall.  Wifi Thermostats are great to have, especially for cold winters in the Northeast where you can monitor for indoor temperatures from you iPhone to prevent frozen pipes while you are away.