I used Dynamic 404 during a major site rebuild. My client was on a defunct CMS and wanted to convert the site to Joomla. The old site had good search engine rankings, with over 400 urls, and we were faced with the challenge of maintaining these rankings after the Joomla conversion. Dynamic 404 helped us to manage hundreds of 301 redirects from the old urls to the appropriate Joomla ones. The result was that we saw minimal negative impact from the search engines, and zero loss in traffic after the Joomla launch. Hi highly recommend Dynamic 404!