This is not a shoe for runners with very flat feet. Several years ago I ran through multiple pairs of an earlier incarnation of this shoe (the NB 855 and 856) and liked them very much.  The new NB 850 has a feature called the Stabilcore that is basically a stiff plastic ridge that runs along the arch.  Since I have a very flat foot, that ridge presses into my arch making the shoe uncomfortable. Many shoes on the market right now have a similar ridge. These include the Brooks Beast, Saucony Guide 3, and NB 1012. Although they all claim to be for runners with flat feet wanting motion control, the design prevents truly flat footed runners from wearing these shoes.

This shoe will probably be very good for runners with medium  or low (but NOT flat) arches wanting cushioned motion control.

I also own a pair of Saucony Guide 3. If you like the NB 850 you might also like the Saucony Guide 3.  The Guide 3 also has a ridge that hits my arches. To me both of these shoes feel quite similar. Though the NB 850 feels spongier than the Saucony Guide 3.