Why is X mirror not working on a mesh? Does X-Axis Mirror work on a mesh?  I applied X-Axis Mirror to a set of bones and it worked but not on the mesh, I tried to mirror the mesh using the X-Axis Mirror option but it doesn't mirror anything. 

Here's the .Blend.
The mesh isn't exactly symmetrical.

This behavior is intentional. Meshes which are almost (but not quite) symmetrical wont detect vertices as mirrored.This is needed to avoid problems with high-poly meshes where vertices may be very close.

To resolve the problem, you can use the Snap to Symmetry tool.
See: Mesh -> Snap to Symmetry.

This has options to select a distance threshold and choose which side of the axis to use.