Stable as a mountain. Fast as a ferrari but has some issues with beta sound and video drivers playing games that rely on BINK Video technology. It presents itself as Server 2003 64-bit so standard desktop apps dont always install and play (I've heard of a game or two saying they dont run on server versions too).
I'm sure you've seen this sticky thread. I have not yet installed it, but am wanting too. From all that I've read here and other forums most of the reviews are positive but many don't see any real benefit as of yet to upgrade.