Product review (5 stars): Great replacement!
My Aprilaire 550 humidifier (installed in 1994!) stopped working at the beginning of this season. I found that the water valve was no longer working. I was amazed to find that a replacement valve was still available. I bought and installed it. (Took some finagling, as the mounting was a little different than the original.) Works fine - humidifier is keeping the house air correctly moisturized.

Don't forget to order the inline filter and the flow restrictor.