Here's a must-read article at Musings From Mars: All The Lovely Browsers! To me it was a real eye opener, and you owe it to yourself to examine the findings.
It covers a range of browsers for the Apple and Windows platforms, in terms of their functionality and speediness in carrying out various types of operations. Safari comes out well, as do Firefox and especially the Opera browser. Internet Explorer (IE6 as well as IE7 Beta) stand out -- by being significantly slower in the tests!
I've been running IE7 Beta 2 for a few months now. IE7 has been very stable, but I cannot put tolerate its (to me) quite unusable native layout and lack of tailorability. So instead I use the excellent Avant Browser front end, and this nicely covers nearly all of my functional needs. All the same, regularly I hop over to Firefox to take advantage of various functions that it does better. (TIP: there's a handy Firefox extension that allows IE pages to be launched in Firefox, and vice-versa.)
After reading the Musings From Mars report, I'll certainly be reconsidering use of the Opera browser (and with any luck achieving noticeable time savings every day).