Alt + x edit mode
The Symmetrize tool extends blender's native Symmetrize with the ability to mirror custom normals.
Since MESHmachine 0.7, it comes with a flick mode, that allows you to mirror in all 6 object space directions, from a single keymap.
Mirroring custom normals can be very tricky on the center edge loop.
MESHmachine's Symmetrize provides options to deal with this, but they are imperfect and often custom normals will need to be fixed manually in the center region.
Symmetrize will always apply to the entire mesh, no matter what the current selection is.
Keep in mind
Partial symmetrizing is currently not supported. Use Blender's Symmetrize if you need to do this.