The tag-cloud replaces the basic menu – Is this a good idea?

Rashmi asks this of a site that’s using a tag cloud as the primary navigation. Given that I’m currently struggling with trying to balance categorising & tagging – I think that there is a possibility for both, to me, categorising is more static, tagging more dynamic, however, I’ve got to get the balance between them & not just repeat the same thing in both areas. However, while a tag cloud can be “fun”, I’ve yet to be convinced about it as the prime navigation method!