Purple Air, talks about how it uploads everything to PurpleAir.com, but their website is really really janky. Luckily, the device has a web server, and spits out data in JSON, so even if Purple Air goes belly up, your sensor should still work.
I have mine integrated into http://www.weewx.com/ via https://github.com/bakerkj/weewx-purpleair , and I calculate an Air Quality Index from this data using https://github.com/jonathankoren/weewx-aqi .
Ideally, I'd like a sensor that calculated all the other pollutants you're supposed to track, but this is what I have.
The sensor is responsive. When the smoke filtered down from the Mendocino Complex fire last week, I saw a huge spike across the particulates, and even today with the Spare The Air alert, the AQI is quite a bit elevated.
The one thing I'll note about its performance, is that after installation, the sensor read abnormally high until the first rain. After that, its performance is on par with other surrounding sensors.
Would I recommend it? I don't know. I feel like I should have shopped around more for one, so I can't really compare it to anything. I do like how it reports data in an open manner.
I compared it with a friends much more expensive device, and the results were within 5% of each other.
There's probably opportunity in the space for someone who can create an effective, reasonably-priced model.
The neat thing is that it has an API, so I was able to build my own pretty dashboard showing PM, VOCs, and CO2 in my apartment.