They're fine for knowing that something is going on, but not great for knowing exactly what the cause is.
For example, when Facebook's services went down in October, people were reporting that AT&T and other cell carriers were down because they couldn't open the apps. As far as I know there wasn't an outage with any of the carriers that day.
I think they’re about as useful as any anecdotal data out there. Unusually high numbers of reports when you’re seeing issues yourself is about as good as it gets until a status page is updated (which it thankfully has been finally).
For example, when Facebook's services went down in October, people were reporting that AT&T and other cell carriers were down because they couldn't open the apps. As far as I know there wasn't an outage with any of the carriers that day.