As the title says, a MUST HAVE component for any website. Fantastic support, good documentation and plenty of updates as the web world changes. I've used it with K2, Kunena, Jomsocial and standard Joomla articles. Even for common components like JComments that don't have an extension, the component is highly configurable to help make SEF URLs for this content.
The automatic Sitemaps for Google, Yahoo, Bing etc are a blessing for anyone with a website with a high turnover of content.