The Importer plugin is a necessary tool when developing for WordPress or moving a site, although it's not without its share of issues.
75% of the time it just works and all content/attachments will import correctly, but there are times when importing a file that is too large, or trying to import attachments from a remote server causes errors. This isn't a problem with the plugin per se, but with the web server that can usually be resolved by tweaking the server config (if possible).
Another issue that I've run across is due to a bug in the Safari browser which causes the importer to hang when importing a file. This is actually due to a Safari/Webkit bug that's been around since 2005 (https://bugs.webkit.org/show_bug.cgi?id=5760), and you can save yourself the headache by doing the WordPress import with Firefox or another browser instead.