Yes, it's reliable and very loud. And I'm thankful for the loudness every time I had to use it, which is at least a couple of times per ride.
The beep that it does on it's own is more of a short chirp, with the loudness of the chirp when you arm or disarm the alarm function. Mine will give that chirp once in a while, as frequent as once or twice within an hour. And sometimes no chirp for days, then I go touch the bike and it will give a chirp. I can live with the intermitten chirp, but other than that, the horn is awesome.
I asked about that chirp here before when I first got the horn but no one replied so I assume it was just mine acting up. But I guess others are seeing the same thing now as more people are getting their hands on it.