I can only speak for the Chrome/Safari WebInspector, and they're good, but I find myself going back to Firebug if I have to do any serious debugging - personally I find there to be a big step up in usability with Firebug meaning I can work faster.
It'll be interesting to see what happens to development tools when Firebug goes cross-browser.
It'll be interesting to see what happens to development tools when Firebug goes cross-browser.