Loved the WWDC 09 build of SL except for one thing:
Anytime I started up the computer or woke it up from sleep mode, Airport would not connect to the Internet and would just have a "self-assigned" IP address. The only way to fix this was I would have to turn the Airport on the computer off and back on - sometimes several times - until it obtained a correct IP address from the Airport router.
I am using both an Airport Extreme G router and Airport Express G router, both had the same issue. But I don't think it was a router issue. After reverting back to 10.5.7, it's running normal.
Other than that, the build seemed very stable and really sped my computer up noticeably.
Early 2008 Black MacBook, 2.4 ghz C2D, 4GB ram