I've been using Chrome, like most others.
I switched to Firefox temporarily since I thought Chrome let through a couple of viruses, but apparently that was just an MBAM false alarm. Firefox 4 was nice and very, uh... Chrome-like but it was a little funkier so I switched back.
Firefox and Opera are good in that you can move around the buttons anyplace you really want, while Chrome has more of a fixed UI. Opera was also the birth of speeddial (the same as Chrome new tab page), which I absolutely hate because it never gives me the right sites. I just use a personal page for new tabs, thanks to the magic of extensions.
IE9 has also become more Chrome like, and Microsoft finally realized that adding more random/useless buttons does not make users happy. However, I never use IE9 b/c of security concerns.