I accidentally ordered two. They came charged. Did not require the 24 hour calibration cited in the manual. I put it by a window fan to calibrate for 10 minutes. Sequentially, they both settled at approx 405ppm CO2. I recently purchased another AQI meter from TempTop so I was able to compare pm2.5 to some extent.

I've only had these a couple days but I've closely monitored and corroborated the circumstances. All measurements between both units are effectively identical, no variance unaccounted for. The CO2 measurements proportionally increased based on quantity of occupants in my office, if the door was open/closed, central forced air ventilation (A/C system), and window fan. Most observed ranged have been between 400 and just over 1000, though intentionally breathing on it will exceed this.

The PM2.5 measurements correlate with the TempTop, though PM2.5 and PM10 jiggle up to 003 over the base reading, which is negligible. Because I purchased these to fill the CO2 gap the TempTop does not capture, I don't have much more to contribute in these readings yet.

The temperature and humidity are in line with all my other devices as well.

There was a review regarding them recalibrating on their own. This is an optional feature if you read the book, though I don't know why someone would want that to be recurring regularly.

I said I accidentally purchased two... I think I'll get more. Maybe one for me, but others as gifts. I think it's valuable to know the quality of air being respired, especially in places you spend the most time.

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Update:
I purchased a QuingPing monitor based on other reviews I read, largely because it esthetically enumerated the measurements I was most interested in, being Co2, PM, and VoC. This revealed that I absolutely needed to calibrate these units... I assumed when I started them and they hung at 500 for awhile, that was a calibration phase; Anyhow, after calibration they are coming congruent with the QuingPing. Even uncalibrated, bringing this unit to restaurants and my vehicle, etc still demonstrated increased Co2 and PM values, that even if the "floor" was high, was still valuable when taken relatively.

My opinion is that the Co2 alone is an excellent mechanism to measure ventilation in typical scenarios, and that can be used as an effective proxy for air quality generally. This assumes average room area, outdoor air quality, and no extraordinary sources of PM/VoCs like aerosols, flame (candles, gas appliances), or a operating workshop. For the budget minded, or entry level gifter, the Co2 only monitor would be good.

The construction of these are solid, even rugged, and the battery life has been at least adequate for my exploration. I appreciate the USB-C ecosystem. The color added to the screen, though it is static and never changing, makes it passable.

I'm probably going to move to QuingPing primarily, and use these as indicators suggested above (maybe in my car and/or kitchen). The QuingPing, while almost twice the cost, is quite featureful. It includes Wifi connectivity, with onboard and cloud telemetry logging, a nice touch screen and interface, is even more esthetic, and common home automation platform integration, etc. I'll probably settle for two QuingPing units, one in the bedroom and the other in my office. I'm still evaluating the accuracy before committing to a second unit, but it's looking promising.