Product review (3 stars): Caveats and then some more ....
(SEE UPDATE BELOW)
Original comment:
I returned this before even setting it up because it looks like a half open grapefruit in that the back of it is in the shape of a large sphere.  THERE IS NO WAY TO WALL MOUNT THIS ONE.  I should give it a 1 based on the fact that no photos are shown of the device from the side, but this is the fault of either or both of this seller and Amazon and not the mfg. CO2Meter, which has a good reputation.  So for mounting this, think of trying to hang a half cut open grapefruit on the wall and that's what you would be dealing with.  For desk use only!

******* Update:  After originally sending this model back because of the physical size of it, I tried a competitor's model, the Autopilot Desktop which was wall mountable.  It claimed to have all the features of this unit plus a graphing feature.  The packaging of the Autopilot was great but the high CO2 setting of the Autopilot would not create an audible alarm as advertised, so although originally  I didn't like the physical characteristics of the AZ-400, I decided to settle with  this unit as a final choice because it is "the only show in town" that is full-featured and in this price range.

The AZ-400 has all the features you can ask for (currently) in this price range, including an easy to read scale, a wide range of detection (can set up to 5000 ppm maximum as a high alarm setting) which is what I was looking for.  Other models for around $100 US only read up to 2000 or 3000 ppm max..  These low ppm models are not really useful for monitoring typical environments where the level of CO2 can be dangerous, such as my own which includes both beverage equipment and a space heater in a small and mostly confined living space.  My testing was done in around 300 sq. ft. bedroom area. The following is a summry of my normal living levels:
Open windows with airflow - 389-450 ppm
Sleeping with all windows closed - 800-1400 ppm
Residing with window cracked and propane space heater - 1400 - 2500 ppm
Infusing CO2 with soda machine 800 - 4200 ppm in bursts as soda infused
(back down to ambient level normal levels listed above in a few minutes)

So because of these readings, I have set my warning level to 5000ppm as a point that I believe a wake-up bell is necessary to resolve the issue at hand (gasp, gasp).  Note that I don't sleep with the space heater on and I always significantly crack a window prior to lighting it, so mistakes won't be made.  However, there is always a chance of falling asleep with it on and having this meter is a great relief for me personally.

Now the bad.  Yes this device is every bit as hard to manage as I had originally imagined.  Just like a tennis ball in a dog kennel, it can't seem to just rest where you place it..  Yep, no matter where it sits, it always wants to go rolling on the floor due to the round ball-like design.  And this point brings forth another issue, the growing amount of these in  "used condition - good" listed in the Amazon Warehouse right now.  In fact this is the strangest item I have encountered  for sale recently at Amazon and in a couple of different ways.  First of all, the stock of this "used condition - good" seems to be growing in the Amazon Warehouse at a disproportional rate.  Secondly, with Amazon switching this item just this week (Apr 2017) to "Prime Only", it's almost as if they are trying to unload the ones in the warehouse.  Add to this mystery, why are so many used ones for sale with almost no bad reviews?

 Which is a great seque for me to describe where I have been the past couple of weeks.  For some reason when I went back to this model (rejecting it initially due to the shape) when returning I went for the Amazon Warehouse deal,  The first one I received would run about 4 hours and then the display would lock up.  It would then require a reboot.  Then, foolish me, I ordered the second one again from W.D.  and the second one would reboot automatically anytime you  just lightly tapped it anywhere.  So "used - good" number two went back to the warehouse.

Then once my two W.D. returns were processed, I decided that the W.D. was more a testing-bed for returned items than it was a place for a good deal.  I then went to order a new version of this one but this time (and only this past week) Amazon has switched it to "prime only".  Well not only did that make my eyes bug out in emotional distress (this "prime only" seems to be happening more and more often it seems), I am also distressed to know that this "only game in down" product will likely soon also be be rolling across my own floor as well.  Wish me luck!