I like having the Address bar hidden in Safari. This saves space. When the address bar is hidden, you can press Command-L to show it and enter any URL, and when you press Return to confirm it, the Address Bar automagically slides away.
Furthermore, when you open new tabs/windows, you can enter URLs even though the Address Bar is hidden. This might be considered a bug, though.
[robg adds: How very odd -- you can, indeed, enter URLs with an invisible address bar on a new tab or window!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030618151155299