Seeing this report, I decided to test it, and have taken a few barbeque coals smouldering and left them near the alarm.
The alarm goes off within about 1 minute, and the screen shows first 25, then "50ppm" of carbon monoxide.
I also tested the behaviour with a low battery, and it does indeed warn you.
Overall, it seems to be working as advertised - I can only suspect that not all of this model of alarm are bad, or perhaps that the british standards have some strict requirements (like perhaps it needs to go off quicker than 1 minute).
For the $2.50, the safety per dollar seems like the best deal around, and it is still way better than not having an alarm.
If anyone else tries this, beware that once the alarm has gone off, it keeps beeping for about an hour, even after you remove the carbon monoxide...