The dev tools in IE8 are pretty darn good. I still use Firebug as my primary, but when I'm chasing an IE specific bug, the dev tools are pretty solid. I even prefer them to Chrome's.
Webkit's debugger, not Chrome's. I've actually switched over to Webkit's debugger as my primary one though. It ironed out a lot of the nuances that irked me over the last year (inspection of styles didn't appear as code [it now does], you couldn't use command/ctrl + shift + c to inspect an element [you can now], you couldn't delete dom elements [you can now], as well as a myriad of others that were resolved). So far I've found it to be more stable than firebug and laid out/organized better. That's just me though, and to each their own - but if you haven't tried it in the last year give it another shot.
