Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Caveat Emptor

This has been a most disappointing product. I've used heart rate monitors for years, generally from Polar. I bought the Timex because the battery is readily replaceable. Unfortunately, the heart rate monitor has never worked reliably. When I first start the monitor, it shows an accurate heart rate. However, after about 5-10 minutes of exercise, the rate inexplicably drops far below what is real. Timex has sent me two replacement chest bands, which produced the same results as the original. I think the watch is defective; Timex insists that if it registers a heart beat at all, it can't be defective. It's back to Polar for me!