A lot of people insist on avoiding the use of "using namepspace x" in .cpp files. This makes no sense to me, it makes things a lot easier to read. It never seems to cause a problem, and it's trivially fixed if a conflict shows up due to a header change later.
It does if you want to use unity builds at some point down the road.
For my app it took the build time from 25 minutes to 2 - 3 minutes which is critical when doing CI on a lot of operating systems and with a big build matrix.
