The import function for Contacts is incomplete, in that you can't import into custom fields.
The import function for Companies is non-existent and equally important.
Importing Contacts seems to cause existing Companies to disappear (at least sometimes). This is a real problem when combined with the fact that you can't (re) import the Companies...
You should also add a staging capability so that import can be validated before committing - it's potentially highly destructive (e.g. user error on the import can destroy your contact base)

