Product review (2 stars): User Interface is User Hostile
In terms of doing its basic job of displaying information about CO2 level, humidity and temperature of the air in the immediate area, it seems to do its job.  I must, of course, add the caveat that I don't have another device around that I can compare its readings to, but I can say that the CO2 level, humidity and temperature indications on this device do seem to track with actions in a room that you would expect to raise or lower these numbers in the way that it indicates.

However, its user interface was crafted with no care or concern for making it easy or even remotely understandable to operate.  It was clearly created by people who were afraid of spending a few more pennies to put a few more buttons with labels that make sense or to put the kind of all-points-addressible LCD display on this thing that would allow it to spell out its menu options and various air quality conditions with REAL WORDS.

Instead, it displays codes like "E02" to indicate that the humidity is too low for it to measure, and menus are organized in a weird nested system numbered "P10" through "P30" with submenus numbered even more weirdly -- you wouldn't believe what you have to do just to set the time or adjust it for daylight savings time!  What fool came up this?  This is actually worse than setting the clock on even the most inconsiderately designed VCR!

Also, returning to the buttons, although the positioning of the buttons along the bottom edge may add a certain bit to its "cuteness" in terms of how it looks, the visual appeal quickly fades when you realize how all-too-easy it is to pick this silly thing up in a way that causes one of those buttons to accidentally be pressed.  When you do that, things start changing on the display, and unless you're someone who regularly works with the controls so that you have the crazy array of options memorized so that you understand what the weird not-real-words indications are telling you, you have to pull out the manual to figure out how to undo whatever you accidentally did by accidentally pressing one of the very poorly placed buttons.

To top all of this off, and almost as if to confirm that someone went out of their way to design this thing to be especially annoying to use, they printed the user manual in tiny print on tiny pages (pages roughly about 1/3 the size of standard letter-sized pages) making it especially annoying to find the operating instructions you're looking for to do something like set the time (of course, it's under menu P30, submenu P33 -- naturally! -- I should have known!).

To sum this up, it seems to actually do its job -- that's a good thing.  But it's so horrible to interact with that I will happily part with it for a different CO2 meter with a user interface that someone actually tried to make user friendly, or at least not user hostile!!!