From BlenderWiki

Mirror Editing

The X-axis mirror option of the Mesh Tools More panel allows you edit both “sides” of your mesh in a single action. When you transform an element (vertex, edge or face), if there is its exact X-mirrored counterpart (in local space), it will be transformed accordingly, through a symmetry along the local X axis.

So as you can see, the conditions for this tool to work are quite strict, making it hard to use in real life… Anyway, to have an exact mirrored version of a (half) mesh, its easier and simpler to use e.g. the Mirror modifier!