We have a Poolguard inground alarm. I am happy with it.
It has an AC powered in-house receiver for the alarm, which you definitely hear! The unit itself has a 9 volt battery which I replace every season.
I haven't had much trouble with false alarms. Usually it happens during a storm, especially if there has been a lot of rain that has raised the level of the pool. I try to reset the unit during or after a storm before I go to bed to minimize that chance (I think I've only gone running out in the middle of the night in my underwear once to silence an alarm.. great visual).
It can be picky about being removed and then put back in the water, in terms of having the alarm go off. You're supposed to hold the button for 5 seconds, hear a chirp, and then you have 5 seconds to put it in or take it out, but sometimes it goes off anyways. Usually just holding the button will shut it up (but not before my wife comes running from the house!).
But overall I'm happy with it, in terms of not having many false alarms. This is should only be one layer of protection as others have noted, and the best protection is as many different layers as you can do (door locks, gates, alarms, net, safety covers, education, etc).
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Virginia Beach, VA
28K inground, vinyl