Nice upgrade, though it seems to be more of a security and small feature update than a larger release. Of course the changes in IE7 compared to IE6 are huge and are attracting a bit of press, as it should – it took MS long enough to upgrade the browser!
However, for daily browsing, Firefox seems to work best for me. I happen to use three browsers, the obvious two and Opera. Firefox is my “work” browser, meaning that I open all my email, blogging, Feed Reader, Search, and other task-based services in it. IE is my “default” browser – its just that, click on a link in email, opens in IE. I also use IE for additional research and news link following.
Opera is not used as much as FF & IE, its mainly for obscure sites – meaning that if the site is suspect, I paste the URL in Opera and see what happens – this simple trick has kept my IE install healthier than any other I have tried.
Anyway, FF2’s got game. Its not any one thing, just a great combination of usability in a browser. The only plugin that isn’t compatible at the moment is my Windows Live Writer “Blog It” plugin. Its a shame too, because that’s my most often used plugin! 
