I purchased an extra Garmin Heart Rate Monitor to pair with my Garmin Vivofit activity tracker that I use both at work and home. I really like this Garmin monitor, much better than my Timex. The monitor syncs with the Vivofit quickly and is accurate right from the start of my workout until I take off the monitor. The adjustable strap appears to be robust and should fit just about any chest size.
HR data can help you monitor workout effort to ensure that your hard days are sufficiently hard and that your easy days are sufficiently easy. If you're not training with power, HR data is better than nothing. If you have a Garmin Edge, you might as well spend a little more for the HR strap.
Regarding fit, this strap is comparable to Timex, Polar, etc. It has a rubber and plastic body, with an elastic strap. It might feel uncomfortable at first, but once you get going you barely notice you're wearing it.
Do not neglect to spit or apply some other fluid on the strap's conductive sections before you put this on. The strap's electronics need a liquid medium to pickup and transmit heartbeat signals to your head unit. Ambient humidity is also a crucial variable, so you may notice erratic data readings in extremely dry conditions (high-performance/wicking fabrics reportedly generate static, which can compromise the electronic signals).
As a 61 year young rider, I like to know what my heart rate is when I am riding. This can tell you what zone you are in and also gives you your rate per minute. Very helpfull when on a long or tough climb. Keeps you out of trouble. Comfortable to wear. Have not found anything I don't like about it.
Garmin Edge 800 was a powerful tool to improve my training but without a heart rate monitor it wouldn't be so powerful. Heart rate monitor is the key of my training nowadays.