Super simple to use. I don't have anything professional to calibrate it with, but it's reading a perfect 65% humiditiy with the boveda 65% pack in my humidor which was "seasoned" when I first purchased it.
I've also tried it in another new humidor with the boveda packs and so far it's been reading the same in there also.
It was super easy to get going(just remove the plastic tab in the battery compartment), and came with a magnet attached to the back and another to attach to your humidor or wherever you want to put it. I didn't need the magnet but it's nice to have.
I think I'll be using these for all my humidors in the future. The built in ones are just garbage

I bought three different (recommended) hygrometers on Amazon and then tested them using the Boveda One-Step Hygrometer Calibration Kit, which guarantees a 75% Relative Humidity inside the bag that comes with the kit. This hygrometer read 73% in the bag (see photo) which is closer than the other two hygrometers I bought. And so, not perfect, but pretty darned close, and much better than other (recommended) hygrometers on Amazon. This Caliber IV one stays, and the other ones are going back to Amazon.

Mine's in the middle of a room where the temperature is always between 55℉ and 75℉.
(The outside temperate doesn't vary much beyond that either.)
Overnight, I'll sometimes get maximum readings of 80℉.
One night the maximum was 97℉, another time 114℉.
I've also seen overnight minimums of 40℉.
There is absolutely no way these readings are accurate.

The humidity readings look believable, but hard to trust given the ridiculous thermometer.

I expected more from this unit.

Also the description mentions 3-day averages for humidity & temperature:
I have seen no averages nor does the user manual mention any.

Furthermore the email address supplied for warranty inquiries does not work.

Out of the box, I sat this in with a proven hygrometer that I own and it was dead even with it for about a week. Since then, I've had it in a separate humidor and it's been working great/problem free. Great size and easy to use. Will give you min/max for humidor same as my proven hygromter. Much better alternative to crappy analog ones that come with/are fixed in many humidors. Great buy at a good price.

Accurate and works as intended perfectly. I wish the screen was a little more visible or had a LED lit colored screen. But it does what it's supposed to and does what I need it to do.