Product review (5 stars): Works just fine
Does just what it said it would. I used it to interlock my humidifier to my AC unit. I put it on the fan commom lead so if the fan goes off it locks out my humidifier. It was easy to install and was made to work with the humidistat and water valve.
I am sure you could use it for other interlocks that use 24 volt AC as well. Just two wires connect in series with the water valve.

UPDATE: 1/4/13 It quit working last week end. I replaced it with a differential pressure switch I got on E bay. Don't know what happened to it but it just stopped working. I would use a differential pressure switch as I have yet to see a DP switch stop working if installed and adjusted correctly.