Product review (4 stars): Once you've set it up, it works reliably and easily. And annoyingly (people put ouf a remarkable amount of CO2)...
Takes a while to set it up, but easy to use afterwards. And it's great for bedeviling your friends and associates on a cold night when the alarm goes off--as it probably will if the place is sealed up and you've having friends over. And then what? You live with the high CO2 level or you open a window and refrigerate your home, or run the heater at the same time and get a sky-high gas bill.

So I'm working out how to balance our heating bill with healthy air. This meter is indispensible in figuring that out. We live in a 3 bedroom condo with good window sealing. Most nights I've found that if I've aired out the place earlier in the day, sleep with our bedroom door open (to our cat's delight), and just leave one window open in one of the other bedrooms...and have an electric mattress under us and a small space heater to warm the bedroom early in the morning--we can get on pretty well. We're also going to replace our 40 year old forced-air furnace with a modern one that's a lot more efficient. That'll cost in the neighborhood of $4K, so the cost of the CO2 meter isn't much compared to that.