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* Like the Polar V800, the M400 uses visual tricks to appear smaller than it is. The watch is curved, so the first part of the watch strap is really part of the watch body. This can cause problems for runners with smaller or larger wrists. I have to wear the V800 and M400 over a wrist sweat band as my writs are quite small. (I have no problem with watches that appear to be bigger, like the [[Garmin 310XT]].)
* There is no vibration alert, something I miss.
* I've found the battery on the M400 seems to run low far earlier than the claimed 8 hours. I've had the low battery alert after only 4 hours, though I was using the backlight. I've also found that leaving the M400 in the state where it is ready to start, with the GPS signal acquired, the battery will run down quite quickly. Slower marathon runners could have problems with the battery life of the M400, and it could not be used for most ultramarathons.
* The polar website has some basic functionality, but it has the feel of an unfinished beta release rather than a complete solution. This feeling of being unfinished extends to the watch itself, but at least Polar are giving a timeline for new functionality.
* The M400 will give an estimate of [[VO2max|V̇O<sub>2</sub>max]] if you're wearing a Heart Rate Monitor, but I didn't find its estimate as accurate as the [[Firstbeat]] software used by Garmin and Suunto.
* It's a minor problem, but if you have a Heart Rate Monitor strap paired with the M400 but don't have it on you, the M400 will wildly overestimate your calories and effort.
=What's Missing=
While I don't consider these missing features as 'cons', it's worth understanding the features that are missing compared with other watches.