Product review (3 stars): So far they seem to be working, but reading high about 5% above my other hygrometers.
These were pretty cheap and i wanted them for my instrument cases to measure the humidity in the case.  For that they were cheap enough, 3 dollars roughly each.  As you can see by the picture,  if you cut out a rectangle, they were made to snap into an opening.  So if you have a cheap instrument case, for instance my ukulele case, i can just cut a square and pop these in there and i'll look pretty cool.  The bad,  well as you can see they're made to be popped into something lol.  And they're stuck in centigrade,  i didn't look close enough to the pictures to see the C symbol or i would have asked the question, but figured most electronic items are switchable between F and C, but this doesn't look like there are any switches on it whatsoever.  So not really using for temp, i'll give it 4 stars for now, as i pulled them out of the box.  They're about 5 points above my cheap alarm/hygrometer/temp clock,  but it should stableize within an hour.  I will downgrade them if they stay high.  Oh and my clock hygrometer measures roughly the same with my d'addario music instrument hygrometer, so if these cheap black tool hygrometers work, i'll let you know.  Save some money if you need to hydrate your guitars,  wood instruments or even cigars.  Oh and finally,  the back has a sliding cover for the two R44 small batteries, and it ships from china so give them about 3 weeks.

Update 1:  so far they've been reading high,  my d'addario 56% my clock 57%  these black tool hygrometers 62%.  All 3 that i purchased are registering 62%.  Maybe they were made to be put in a case or mounted so they're more sensitive?  I mean i can live with it knowing it's 5 points higher then my other 2 hygrometers, so i still can use these.  But i'll check again tomorrow and see what the readings are.  Oh by the way,  the humidity weather readings on the weather channel say it should be around 60%, so if that's true, these are off by 2.  But i think i believe the other hygrometers as they're sitting in my living room on the table.  So i expect it to be lower then the outside.  But i will post another update in a few  days.