I've been using IE8 since it's official release. Microsoft recently started pushing it as a recommended update.
Think Firebug for Internet Explorer.
IE8 includes the Developer Toolbar by default, which can be activated by pressing F12.
The new tools inlude the usual DOM and CSS explorers as well as the new debugger.
The debugger interface is pretty straighforward and should be easily accessible to anyone who has used an interactive debugger.