I'm all for bleeding edge tech, I mean I AM using Server 2008 over Vista or XP haha, but pre-RTM Service Packs are one thing I have NEVER touched and never will!
A service pack is designed to fix bugs, possibly add new features, and generally improve the experience. But, being Beta, there is a very high chance that it will in fact have an OPPOSITE effect - making your system more vulnerable, more unstable, and maybe even slower!
I strongly recommend that if you DO download this, please don't put it on your primary OS. It is PRE-RELEASE software! Yes I know you can uninstall service packs, but seriously, if you are not a Tester, or if you are not extremely bored with nothing else to do, just wait for the RTM/Final release