I think there's a lot of things to be considered with this!

If you put it into your personal.xls file, no one else can use it.
If you put it in a workbook and save it as an xla file, then when
users open the "add-in", excel  copies the file to their local space.
This is a problem if you plan to upgrade, add  to, or improve the macro.
The users of the add-in will not receive the changes.

another option is to pursue defining the commandbar in vba, then
putting the macro for the command bar  in the Workbook_Open event,
or the Auto_Open macro.