I use the CO Experts type that is mentioned in
Mike Busch's article.
It is very sensitive and goes off at 20-30 ppm that I get when taxying with the canopy cracked open. I make a point of reassuring the passengers that it's OK.
I was, however, very glad when it warned me of an exhaust failure bringing CO into the cockpit. Turning off cabin heat and opening the cold air vent soon cleared the cockpit, but without the warning there may have been a different outcome (the alarm beeps were quite audible in a very noisy cockpit).
The card types are not great. Short lived, no alarm and sensitive to other contaminants besides CO, but certainly better than nothing.