I haven't used it yet, only have the PM2.5 hooked up right now. But for what it's worth I grabbed the Adafruit SGP30 breakout. I'm not going to be "happy" with it, since it's not a real CO2 sensor, but I don't have $1000 for a real sensor. Hopefully it'll be interesting regardless. The PM2.5 sensor for all its faults has at least given me a better idea of what activities around the house cause increases in particulate matter. It's not useful information, but it's fun and interesting.

I'm using a co2meter.com one [1] connected with USB. Reading out the values is a bit slow, but I just do that in a cron job every 5 minutes and log the results. This does exactly what I needed it to do with a minimum of fuss, so I'm happy with it.

[1] "CO2Meter RAD-0301 Mini CO2 Monitor" on Amazon, $70

How great would it be if Apple built CO2 and particulate sensors into their products.