Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Unreliable at best

For $32 along with a fair amount of great reviews, I was pretty excited about this heartrate monitor. But I found it worked only some of the time. When it wasn't giving a relistic reading, which was about half of the time, it just read "0". Not helpful at all.I exchanged it for another one (really pleased with Amazon actually how easy and fast that was) and the second one was worse than the first. It only managed a realistic reading a couple of times, mostly it read zero and once it read extremely high for the activity level which was a bit alarming. Very weird. Had to face the fact that it's pretty useless, return it, suck it up and pay more for a different model.