Here's my take on browsers.
I'm not particularly thrilled by either IE or Firefox anymore, but I use Firefox 2. The Adblock and Filterset.G extensions are the only things keeping me loyal to Firefox right now. It would simply be impossible to surf the web now and look at ads.
Were Adblock/Filterset.G available for Opera 9, that would become my browser of choice. Opera seems to be faster than Firefox, and its user interface is a lot cleaner. Additionally, it has built-in support for user scripts which can modify certain web pages on-the-fly. For instance, a script is available that allows you to save photos directly from Flickr, circumventing their method to prevent you from saving pictures. Firefox has this capability also, but only via extensions such as Greasemonkey.
IE7 seems to be pretty secure, but the changes in the interface have made it almost unusable. The organization of the menus and toolbars flies in the face of 10-15 years of software evolution. Microsoft should've at least included the ability, as with Firefox and Opera, to customize the toolbar buttons and their positions. It takes a registry hack to put the menus back where they belong. Nevertheless, I have upgraded to IE7 on my computer, if only for the improved security on the few occasions I use it.
It will be interesting to see how things evolve again when Windows Vista releases next month and starts spreading. Vista claims to be pretty secure, and makes a formidable combination in that regard with its version of IE7. MS has already "agreed" to play nice with Mozilla and assist in their development of Vista versions of Firefox, Thunderbird, etc. Hopefully they stay true to that agreement, and agree to help Opera's developers in the same way.