One of the oldest devices I got for CO2 monitoring is AZ-7788 GreenDay. It is 4 years old now. The device is still good and accurate.
Used as portable mostly because it has an LCD and not require setup.
This model got a relay to switch on/off some ventilation device (never use that). Other models 7787 (Green Life) without a relay and 7798 (Green Eye) with data logger available. See the list.
Will not go for cons/pros. Since this is kind of reference device I compare other devices with.
The device has old Senseair K22 sensor inside.
Green Day inside: see wiki
Price in Ukraine: 4680 UAH (180 $)
AliExpress: 130 $