Now that the NDA is over, reviews are starting to pop up across the web. So far, the one by Engadget is the best IMHO. It covers some of the things people aren't talking about, such as compatibility issues. It seems like this upgrade to 10.6 is going to offer the most advantages, but offering a lot of compatibility issues with some of your favorite apps -- until they release appropriate updates. Even though 10.6 is defaulting to booting 32-bit kernel to offer the most compatibility for drivers and applications, it's not going to be as easy of a transition from Tiger -> Leopard. If you want to find out more about this and get the real lowdown, check out the latest episode of The Typical Mac User Podcast. My friends Victor Caijio and Steve Stanger go over what you need to know before you jump in with both feet and upgrade.
I am definitely upgrading Friday, but for the cautious, you may want to give it a week or two.