I upgraded to Snow Leopard yesterday, and it is fantastic. Noticeably snappier in every respect. The Exchange 2007 support is great, it took a while to initially sync all my mail (mass quantities), but after that it has been singing along. Address Book integration is nicely done, it doesn't try to merge, it keeps your accounts separated, but provides an All Contacts view. GAL search works too (and in mail compose).
Snow Leopard comes with 64-bit Java 6 out of the box, woohoo! I loathe Java, but as I'm working with it now, I am very grateful for the massive speed upgrade, and the JDK 6 support.
I installed MySQL 5.1 x86_64 and it runs perfectly. The PrefPane is 32-bit and requires an (automatic) relaunch of System Preferences, but that's no big deal, it is used infrequently.
*Everything* is faster. So far the only casualty is Checkpoint SecureClient, which I knew in advance.
Good stuff, Apple!
OH!! I also got back almost 15G of HD space! LOVE.