This watch is one of the most highly recommended basic, no-frills, heart rate monitor. Despite its low price, this HRM watch gets consistently good ratings from many websites, including several jogging magazines and the Consumer Search website.
This HR monitor watch comes with a chest strap transmitter which measures the rate of your heart, and is more accurate (link to types of HRMs) than strapless monitors. The chest strap transmits HR measurements to the wristwatch, which can display either the time or the heart rate. The wristwatch can also be programmed with high and low HR alarms, so that you hear an audible alarm when you stray from your target HR zone. Unlike many other models, the battery in the HR-100C chest strap is an ordinary CR2032 watch button battery, and is designed to be user-replaceable. Many users who just want to know their heart rates when they exercise are very satisfied with this basic device.
HR-100C Complaints
Complaints about this watch tend to focus on 2 problems. First, the transmission from the 
chest strap is not coded, and interference can happen if your exercise takes place in a gym with many other pieces of electrical equipment. Secondly, some users have complained that this Omron watch became erratic and died after 3-12 months of use, although the company says that this is probably due to the strap battery running low, and should be solved with a battery change.
Overall, however, the vast majority of this Omron HRM watch users are happy with the HR monitor, and it is one of the more popular basic models on the market.