This is a decent CO detector. For anyone that has used any 9V battery backed detector (be it smoke or CO), the long life lithium backup is wonderful. After 10 years, you have reached the operational timeframe of this meter and should replace it. So, as long as you do not have excessive unplugged or power off time, you will never have to replace a backup battery. That is great. The overall footprint of the detector is the same as Kidde's 9V backed up detectors.
For those with horizontal plugs, the unit also allows rotation of the plugs to keep the monitor in the proper orientation.
This detector has a display on the front, showing CO in parts per million. The accuracy is plus or minus 30%. As with most meters of this level, you are not going to get great accuracy and very little at close to zero. However, once you get around discomfort and danger levels, it does its job. Just understand that despite the 0 on the front, UL2034 does not require testing for CO lower than 70 ppm.
Using canned CO, I found the response to be acceptable. It takes a while to get a positive result, as indicated in documentation: 60-240 minutes for 70 ppm, 10-50 minutes for 150 ppm, and 4-15 minutes for 400 ppm.
It is not a good solution for those sensitive to low level CO issues, but for most people who are looking at dangerous large failures of combustion based systems, this is a good choice. Detectors that can accurately read lower levels are generally about 5 times the price. So this sits at a value point that most people can justify spending.
Battery Length, Nice Design