I really like Safari 4 so far. It's sleek and fast. In many ways, it reminds me of Google Chrome. Unfortunately, the F11 shortcut does nothing in Safari 4.
Will Safari 4 become my main browser and replace Firefox? No. I rely too much on Xmarks (formerly Foxmarks) to synchronize my bookmarks/favorites among multiple machines. Although Xmarks is available for Safari 3 running on Mac OS X, it isn't available for Safari running on Windows.