Product review (5 stars): SIMPLE BUT EFFECTIVE
This device is laughably simple but could be useful if you're unable to monitor your freezer, like if it's a freezer in another location that you're not around all the time (cabin, etc).  We have a big upright freezer and the freezer on top of a second fridge in the garage so we're always around and would know if anything wasn't right.

The way this works is you fill it with water to the designated level and then freeze it.  Once frozen, you put the plastic ball on top of the frozen water in the tube, and put the whole thing back in the freezer.  What it will show you is if your freezer lost power and the restarted again.  It does that by the fact that the ice in the tube melts and the ball will sink into the water.  Then if the freezer restarted and froze things again, you'd see that the ball was no longer at the top, but was frozen somewhere down in the water.  If you fine the ball frozen at the bottom of the water, then it indicates you should probably throw out whatever food you have because it's been unfrozen for too long.  That's the concept anyway.

I haven't had the chance to test the unfreeze/refreeze aspect as neither of my freezers have failed while this tester was in them, but it seems like it would be a good indicator that your freezer had lost power at some point even it if appears to working when you look in on it.  I'd put it in a cabin freezer or vacation home or even an RV.