"For the great majority of the users we encounter, Visio provides the most economical environment to create, maintain, and deploy diagrams and drawings. A small percentage of potential users have highly technical needs for which CAD systems are better suited. In cases where direct and complete file compatibility is critical, CAD may also be a better choice. For everyone else, Visio has more than enough capability to support the drawing process and integration with corporate data."
It make a great comparison of Visio and AutoCAD and dispels some myths about Visio. These guys has been in the business for a long time and are well respected in the Visio and CAD community, be sure to give their entire article a read: