Product review (4 stars): Very helpful when installing the Aprilaire Whole House Humidifier
After receiving my Aprilaire 600 (or whatever model it is) I realized I needed a bunch of parts to get it installed.  I checked on Amazon and sure enough this kit existed.

I've given it 4 stars only because it had everything needed to put the unit on my system except for 2 screws which are required for installation of the control panel to the duct.  The two screws to mount the control are slightly longer than screws provided to mount the unit.  The kit included way more screws than were necessary but failed to include the 2 longer ones.  A short drive later and a few cents in screws had the whole system installed.  I was surprised how easy it was to put in having never done any ductwork before.

I ended up using some foil tape to seal up some of the pipe pieces and you may want to pick up a roll before you get started on this job (along with those two screws!)

The biggest pain in installing the system is figuring out where and then drilling the hole for the external temperature sensor.  Hopefully the best place isn't very far away from where you're working!