Product review (5 stars): Keep it OEM.
Our HVAC setup is about 17 years old, and we're the second owners of our home. Our system has an Aprilaire 550 humidifier attached to it, and while everything has been working fine, I've essentially rebuilt the humidifier in my latest round of preventative maintenance. I purchased this solenoid as well as the Aprilaire 4793 maintenance kit. We have a water softener, so there weren't any issues with build-up on the original parts, but I feel better knowing what has been done on it. These things were easy to install, and I am as novice as they come. There are plenty of online videos to assist with these things, if necessary.  Now all I need to do is replace the water panel every year, and we should be set for a few more years.