At first, all I could think was: WTF?? Why waste precious engineering resources on this??
But after the announcement of the "iPhone SDK", it all makes sense: they need a Webkit-capable browser available on all platforms to make it easier to test future "iPhone Apps".
That small detail aside, Safari3 is working ok for me with the latest Mac OS X 10.4.9. Its very fast, and for now stable.
It has some goodies like SVG support that are nice (you can play Tetris, or see several other demos) and the Safari debug palette is now built-in (right-click, and select "Inspect element..."). Its not something new, everybody who uses Firebug knows about this, but its nice to see it built-in into Safari.
For now, it will be my preferred browser, but I keep the uninstaller handy. We never know.