I just wanted to post that we've been spending the past 3+ hours dealing with the extremely beta Importer module. We've narrowed down a major cause of our pain: the importer is terrible at handling anything to do with deleted users. On top of that, the error reporting of the module leaves a lot to be desired. I'd hope that extensive time is put into the importer as it is the first experience a lot of paying users deal with. Our upgrade process has been miserable.