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=Polar M400 Pros=
* The user interface is nicely designed and intuitive; the buttons, display, and the menu system combine aesthetics with usability. The M400 has five hard buttons, which I much prefer over a touchscreen interface, especially when wearing gloves or in the rain.
* Like the [[Suunto Ambit2 R]] and [[Polar V800]], the M400 can be configured via the website, which is easier than fiddling with the watch itself. Most of the options can be also set on the watch, which means you're not stuck if you're away from the Internet.
* The M400 provides more information when you press the lap button than most other watches.
* The M400 can act as a simple activity monitor, but it only has only an internal accelerometer which provides poor accuracy. If you need an activity monitor, I'd recommend the [[Basis Activity Tracker]] which has sensors for heart rate, skin temperature and perspiration.
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=GPS Accuracy=
The M400 has the worst GPS accuracy of any device I've testingtested, worse even than the Garmin 620 was before the firmware fixes improved things. * Both Trueness and Precision are remarkably poor. The M400 is nearly always giving a shorter distance than I actually ran, over 250 feet/mile short on average.
* The M400 does a better on repeatability, which is a measure of how likely it is to give the same indicated distance on a particular part of the course. This tends to give an illusion of being correct, as it is easy to mistake consistency for accuracy.
* An out-and-back turnaround is challenging for any GPS watch, and this often shows up the difference between a good and bad a device. A really good GPS watch will typically be out by 2 to 3%, but the M400 does particularly badly with an error of over 13%.