I have always had every other browser installed on my desktop but either out of being lazy, for speed (real or imagined) or since my work involves ensuring supported websites work efficiently with the most popular browser, I default to IE 80% of the time.
That was until I installed IE9. The UI is horrible what were they thinking, it is a downgrade from previous versions. I figured I would get used to it but it is just too buggy, crashes and does not render sites properly. So now I find myself always switching to Firefox. I think IE9 finally broke my addiction to IE. Seems they went totally down hill after IE7.
I am probably in the minority here since overall believe the UI for Microsoft products overall has steadily gone down hill and now mirrors their competition in the 90s. The easy of use UI and training folks was much easier back in the days of XP and 2003. The UI for Office 2010 and Windows 7 menus and explorer blows chunks just like IE9.