This is the my first Heart Rate Monitor and as the M4 is relatively new with few reviews I took a chance on it. I have used it now almost daily for three weeks at the gym. The wrist watch is rather bulky and thick but comfortable and with large numerals. Yes it is also a watch, something Suunto does not push in its advertisements. I find myself wearing it to bed, the only watch I have ever done so, and with a press of a button it lights up and I can make out the time without my glasses.
I do not use the fancy exercise modes, calories burned, or the fitness test so I cannot comment on these. I doubt the calories burned has any accuracy to it but it might make you feel better about yourself. I bought this HR Monitor for just that to accurately give a me a moment to moment account of my heart rate when I exercise. Being recently diagnosed with exercise induced atrial fibrillation I have to be careful to keep my heart rate below my usual threshold of 140 beats per minute. For this the watch performs flawlessly. I actually watched on the monitor as I had a 10 second run of atrial fib with a rapid ventricular response. I stopped exercising and my heart rate suddenly dropped 40 beats per minute. In three weeks I have learned what my limits are and can usually keep my rate between 100 and 130 and have adjusted my exercise routine accordingly.
The M4 is not cheap but the quality is well worth it. The chest belt is very comfortable. I strongly suggest you go to the Suunto website and review the "how to" movie "before you try to use the watch" as this was a time saver. The Suunto "Quick Guide" paper you get with the watch is cryptic. No need to wet the contacts on the belt. Put it on and in two minutes your normal sweat will provide the moisture and you can then press the button to pickup your heart rate. A very good product!!!