Basically, none of them are perfect. I have IE7, Firefox and Opera. There are sites where one or the other of them makes the page look funny, or even unusable.
In terms of look, I like the sleek IE7 look most, but the problem is this look is ruined if you have even one custom button (eg del.icio.us) as it appears in a bar all by itself. Opera has a very good look as well.
In terms of speed, Firefox is probably quickest for mine. IE7 seems to be a bit slower, and even more so if you turn on the anti-phishing functionality. And Opera just seems slower again.
I like the way Opera has the "Top 10" tab which appears whenever you click the URL bar, which shows you the 10 most frequent links that you've accessed. The way Opera's bookmarks are listed are horrible however. The space between lines is so tiny that the mouse cursor often loses its place when you're drilling down levels.
In short, none of them are perfect.
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