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Debian Unstable is the unstable branch of Debian. It has the latest package updates, which ensures that the system is always up-to-date with the latest software. This freshness of packages can sometimes cause some bugs that require manual user intervention...
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Con Not for users new to Linux
You need to know how to maintain Debian Unstable or you can easily bork your system. There isn't very much information available on how to maintain Debian Unstable either. I was fortunate enough to be involved in a now defunct Linux distribution where a forum member put together a how-to on how to upgrade Debian Stable to Debian Unstalble, as well as a separate forum where users could post tips or ask questions about Debian Unstable. One source I would recommend is the Siduction forums.