Seems to work well in the following:
Firefox 6,5,4,3
Opera (Latest version)
IE8 9
Chrome (Latest)
Safari (for windows)
The only issues I personally have had, and I'm sure this is isolated to me is with firebug (latest) all the way down to 1.73 with versions 5 and 6 of firefox with overlays hanging, popdown menus not loading but as I said this is down to maybe the add-on and many people have reported no problems themselves so the above, atleast for me, work well without issues.
Haven't tried earlier versions of IE 6 & 7 but i shouldn't think there are major issues with them using the xenforo product.