Product review (5 stars): Excellent part (and far cheaper than a repair man fee!)
About a week ago I noticed that when my furnace was off...my Aprilaire humidifier just kept running. Well, to be more specific, the water flow into the Aprilaire was remaining on.  At first I feared the control system in the furnace was failing to send the "close valve" signal. But a bit of investigation uncovered the culprit:  the solenoid valve was "leaking"!  When the furnace was running, it dutifully sent an "open valve" signal, and the solenoid fired as usual, sending water to the Aprilaire. But when the furnace shut off (and/or the humidity set point had been reached), the signal to "close valve" was properly sent, but the solenoid did not fully close.  The water supply decreased, but there was significant leakage passed the seat of the valve, causing the water to run down the drain 24x7.

Option 1 was to call my furnace company and have them come replace it.  My guestimate was $100-150 to make that happen.  While they are normally worth every penny, in this case I struggled to justify spending that much for so simple a fix.  So I started snooping on the web and found this replacement part here on Amazon.

I could not be HAPPIER! It took me all of about 10 minutes (and I am NOT a skilled plumber!) to swap out my old solenoid for this one, wire it up, and give it a try.  It works PERFECTLY!  Now, to be clear, you will need some tools.  An adjustable wrench and screw driver are musts obviously, but what some people may overlook is you may need a copper tubing cutter.  When I removed the old one, the old copper fitting would not fit properly in the new solenoid, so I had to cut it off and installed the new fitting that came with this valve.  Other than that, installation was a breeze!

Let me clarify a few items that I saw mentioned in other reviews:
First, this solenoid DOES ALREADY have a strainer on the inlet!  Do not waste your money buying a new one, as recommended in other reviews and as shown in the "bundle" in the ordering area on Amazon.  The filter is built in, and in fact if you bought another, you would be unable to make it fit.

Second, this solenoid DID FIT in the exact mounting holes that the old one used.  Maybe I was just lucky, but I saw others say theirs did not fit.  Admittedly the tubing routing had to be shifted a bit (this solenoid is not the exact same size as the old one), but the mounting holes lined up perfectly.

Finally, YES you must use a yellow orifice (or whatever color is right for your Aprilaire) to restrict flow on the outlet... BUT before you buy one, take out your old solenoid and see if you ALREADY have one that is in good shape!  I wasted a few bucks on a new yellow orifice when in fact the old one was perfectly fine and went right back into service with the new solenoid.

It remains to be seen how long this solenoid will last, but I am VERY happy with this one so far.  With about 10 minutes of work I saved myself probably $100 in repair fees and now my water is not running down the drain 24x7!