When the right conversion is applied -- as it is by default -- PA2 sensors are actually pretty close to the official EPA sensors. Like, they produce the exact same AQI 90-95% of the time, and they're within 5ug/m^3 >98% of the time. They predict the wrong category (good/moderate/UHSG/etc) basically never (<1% of the time) and when they do it's typically because the value was right on the line between two categories.

Even if that weren't the case, though, they fill in a major gap in the EPA sensor setup that no one talks about. There aren't that many EPA sensors out there, but if you go to the EPA website to look at air quality, it will show you a value for everywhere on the map. It does this by interpolating between sensor stations and taking into account weather data. This is often not just wrong, but so wildly wrong that I think it's irresponsible to even show it. The PA2 sensors could be a factor of 2 off the official values and still be more useful than that map, because they're everywhere and they're consistent. They would regularly report dangerous air quality values in regions that the EPA map does not, which is a lot more valuable than being right on the money in terms of the actual numbers (though again, they pretty much are always right on the money).

They're not really accurate to the extent that you'd want the government basing health-related decisions on their readings (I'm not sure any crowd-sourced thing could be reliable enough, either), but they're fantastic for what they are, which is 1, widely distributed and 2, consistent.

Kind of like citizen weather stations, they're not a replacement for the government data, but they're an excellent supplement to it.