Stop Watch with time in Zone (click for stop watch comparison)
Measuring the time you spend in your target zone gives you the quality time of each workout.  This way you can compare your total time and your quality time.  As you fine tune your training you can narrow the target zone especially on harder days to ensure that you are spending enough time going hard.  For example set your zone from 155 to 180 when running or cycling and make sure you spend 15-25 minutes in the training zone.  On easy days set the zones from 112 to 140 and make sure most of  your time is spent in the target zone.

Night Vision
When you touch a button a light comes on so you can see the display in the dark.  The light stays on for 3-5 seconds.  If the light was always on the battery would die quicker.

Time below, In and Above your target Zones  (click memory comparisons)
These features allow you to further fine tune your workouts allowing you to divide your workouts into three zones instead of two.  For most workouts I set the zones so that the workout is divided equally in three target zones.  The time above, below and in the zones will be equal.  At the end of your workout you can compare your projected work out

Average Heart Rate   Reebok    Polar S Series  Polar achiever line  (older models)
Knowing your average heart rate for the entire workout gives you a clear picture of how much effort you put into the workout.   This information combined with perceived effort tells you how your body is  responding to your training program.  In the Polar S Series  and Reebok  heart monitors  your averaged is display each time you press the lap button,  this gives you instant feedback about your workout and allows you to make adjustments.  

Percent of Maximum Heart rate
Cardiosport Advanced,   Autozone  Reebok  Polar A Series   Polar S Series  (advanced)
Program in either your age or your personal maximum heart rate and these monitors will display both heart rate and the percent of your maximum.  Many training programs give the zones based on percent rather than actual heart rates so these are good for people who don't like to do math.

Interval timers (Countdown)   
Polar S Series    Cardiosport Advanced,   Polar achiever line  older models
This feature is great for interval training.  You can set the timers to give you an audible alarm allowing you to break up your workout to your particular needs.  Example 1.   Warm up and cool down:  Set timer 1 for your 10 minute warm up and timer 2 for your 30 minute workout.  After timer 2 is completed the Polar heart rate monitor will go back to count down the 10 minutes from timer 1;  you can use this 10 minute period for your cool down.  Example 2.  You could  set timer 1 for 2 minutes of hard running followed by timer 2, for 3 minutes of easy running.  The timers will continually alternate as long as the stopwatch is running.  If any timer is set to zero the remaining timers will alternate.

Recovery    
Polar S Series  Cardiosport Excel  Cardiosport Advanced   Polar achiever line  older models
This feature is used to evaluate your fitness level after exercise. It is good to set a one - three  minute time frame and see how many beats you recover in that time frame. Compare this recovery heart rate change for each exercise sessions.  Make sure that during the two minutes you keep your activity consistent. For example, you will recover faster if you walk rather than jog.  As your fitness improves your heart rate will drop faster.

OwnZone    Polar M Series

With the OwnZone feature, Polar Smart Edge and Polar M Series  makes exercising in the right level easier than ever before. Not only does it measure your heart rate as accurately as an electrocardiogram (ECG), it automatically calculates the effective and safe heart rate range for you to exercise within to improve your general fitness and health.

The Polar Smart Edge, M21, M52, and M71 use multiple methods for determining your OwnZone. The primary method is to measure and analyze your  heart rate variability. During the warm-up period, these heart monitors listens to your body and analyzes your heart beats. It guides you through an appropriate warm-up routine and automatically determines a safe and effective exercise heart rate zone - your OwnZone. The OwnZone takes the user's age, height, sex and current physical condition,  at the time of the warm-up into account when calculating the target zone.

OwnCal

The OwnCal feature calculates the number of calories expended during an exercise session based on the user's weight, heart rate and gender. Counting of the calories burned begins immediately when the user's heart rate has reached the OwnZone , which is also the most effective and safe heart rate zone for calorie consumption.

You can easily follow the effect of exercising displayed in real-time. The higher the heart rate, the faster the energy expenditure accumulation. Energy expenditure depends also on body weight; the heavier the weight the higher the energy expenditure. At the end of the exercise session the Polar M series heart rate monitors shows not only the energy expenditure of that particular session but also the cumulative total energy expenditure value. Now you know how many kilocalories you consumed by exercising during a week.  In addition the M series also keeps track of how many calories have been burned from fat.

Multiple Target Zones   Polar S Series   Found in Polar achiever line
You can switch your zones in the middle of your workout to reflect different intensity levels or set up the zones for different sports if you are cross training.

Coded Transmission  Polar S Series   Found in Polar achiever line
Avoids cross - talk by coding the heart rate (HR) data signal from transmitter belt to wrist receiver.  When training in groups the Heart rate signal of other athletes doesn't interfere with your own HR signal. This unique feature is only available from Polar Heart Rate Monitors.  Coded transmission also allows multiple signals from a bike sensor or cadence sensor to be picked up simultaneously.  Sark Products let's customer choose transmitter of their choice at no additional cost.  All transmitters are compatible with all watches except, Polar and Cardiosport transmitters do not work with Acumen watches.   And you must have a Polar coded transmitter to work with Polar's Cycling units,  S510, S710, Xtrainer, Protrainer XT Cycle.  Compatibility chart

If your heart monitor watch is 36 inches away from someone else's transmitter, then you will not pick up interference.  In addition your watch should be within 30 inches of your transmitter for continuous read out.