Product review (5 stars): Seems to be better than the original--also, how to troubleshoot solenoid versus humidistat
This seems to have more metal inside than my previous humidistat, even though the cover looks exactly the same.

How to troubleshoot whether you need a solenoid valve or the humidstat (you'll probably need your furnace on to test):
1) Pull knob off of currently installed humidistat
2) Pull cover off of humidistat
3) Either unscrew the two screws with wires attached and touch the wires together, or it's easier to jumper a wire or a screwdriver across both screws to complete the circuit.
4) Your solenoid valve will turn on and allow water through if it is still working, when you complete step 3.

I did all the above and determined my solenoid was working. Replaced the humidistat and everything is working perfectly now.  Will lower my rating if this thing craps out within a couple of years.