Product review (5 stars): Exactly what I needed--saved me $150
I had a heating and plumbing guy come in and take a look at my furnace. He recommended replacing just the humidifier pad, an quoted me $198 to do it. I thought I'd poke around and see how much one of these bad boys cost a la carte. Not very much, it turns out. I think I saw this for $10 or $20 all by itself, but then noticed this whole maintenance kit for about $40 after shipping. Aprilaire's official website had it for sale, too, but for a higher price. I compared the two items and it appears that this kit has the same stuff as the one sold on the Aprilaire site. Either way, when I got the humidifier pad, it came with Aprilaire branded packaging. The other items were more or less loose in the box, but they were all the right parts. As I was replacing the humidifier pad (a $200 job, per the heating guy), I could easily see where the other parts fit into the assembly. I used all the parts in the kit except one. It took me about an hour. The kit came with the same instruction booklet that you can find as a PDF online, which was very handy.

I'll probably buy more of these and do this once a year. Maybe not replace all the additional parts, but at least the pad.