As a natural born worrier this gave me peace of mind. I used it with both pregnancies and managed to hear a heart rate from about 25 or so weeks. I found it especially reassuring if I hadn't felt much movement during a particular day.
The tear-drop shaped device is powered by batteries and is attached to your bump by an elastic belt with Velcro fastening. It isn't especially comfortable but as you only need it for a few minutes this isn't a big problem. The on/off switch is on the side of the monitor and doubles as a volume control. Two earphones are provided to allow you and your partner to listen. I found that it took a while to get the device in the correct position to hear the heartbeat, with my latest bundle of joy it used to change frequently.
The monitor also comes with an adaptor to allow you to record the heartbeat on the computer, I never tried this so cannot comment.
Although this gave me peace of mind that my baby was still alive I didn't have the knowledge to recognise if my baby was in distress. For that reason if I ever felt worried I would still speak to my Midwife immediately.