The new "Nitro" javascript engine makes it up to 30x faster than IE7 and 4x faster than Firefox 3. With its immense list of great features, it will surely kill IE. IE can't stand up to anything in the market now. It's the one catching up.
Since I had my Macbook Pro for the last 9 months as my primary development environment, I have to use its Safari browser to test my web projects. Good thing there's also Firefox on the Mac OSX as an alternative. But as it turns out, developing with Safari + Firefox first was a great move. It's a time saver when I do browser compatibility before I test it with IE.
Now, what I want to happen with Safari 4 is to become the benchmark in the browser market as what the IPhone did in the mobile market for a better web experience.