I have had my Polar M400 Running Watch for a full month now. Prior to purchasing it, when I went running, I was actually wearing 3 devices on my arm: a Timex watch which worked with a chest strap heart rate monitor; another Timex watch which allowed for repeat intervals which I needed in order to use Jeff Galloway’s run-walk-run method; and a FitBit Charge HR. In addition to what I was wearing on my arm, I would sometimes carry my cellphone, on which I would run either the Nike+ app or the MapMyRun app. My Polar M400 replaces all 4 devices — and then does things that none of my devices could do.
To state succinctly what I am going to say about the M400: I love it! What an awesome device! I bought it from Amazon (http://www.amazon.com/Polar-Sports-Watch-Heart-Monitor/dp/B00NPZ7WNU/) for less than $160 (without tax or shipping), and it came with a chest strap heart rate monitor. It was around $100 less than its competitors, such as a number of Garmin products, such as the Garmin Forerunner 220.
In addition to accurately reading my heart rate with the chest strap monitor — something I couldn’t trust the FitBit Charge HR to do from my wrist — it has a GPS built in, which seems to sample all of the data I am interested in every second (since I can download the data in a csv file and examine it at that level of detail). I had not really paid a lot of attention to either Nike+ or MapMyRun until the end of the run (I found it awkward to be pulling out my big Samsung Note during a run), while wearing a GPS on my wrist has enabled me to be interested in what it can tell me during the run. In addition to various distance measures, it can also tell you your current speed and your average speed for the lap you are on and over the course of the run so far.