Is Downtector a reliable and useful source of outage information?

No as it's completely based on users reporting if an issue exists. If a user doesn't report the outage, then it doesn't get caught.

Try ThousandEyes Outage Map -

https://www.thousandeyes.com/outages/

The map is free and it's updated every 5 minutes showing ongoing and recently detected outages.

ThousandEyes actually monitors servers of cloud networks and popular applications for disruptions in activity, which is a lot different than Downdector's reliance on user sentiment.

I use it daily and it has seemed to be fairly accurate for me. I am open to alternatives, though, but I haven't really gone looking for one.