Product review (5 stars): Used it on HCM-350 with no problems.
This was not the same size as our original Honeywell filter, but I thought it was actually much better!  It was about twice the thickness.  This worked out to be a huge bonus as it put a lot more moisture in the air in the same amount of time  The price was also nice...a few bucks cheaper than all the others.  The ones we received did not have the metal screen to hold it up (which was fine with me).  Mine were a little floppy, but so were the ones that Honeywell sent us with the humidifier.  This has never caused a problem.  Would definitely buy again.