It takes a bit to get used to if you're a Firefox 'poweruser' and I'm not sure 'Web Inspector' is quite to the maturity of Firebug yet... Still, it's my primary browser.

One cool thing not to miss is in the unified location bar, it does some search stuff via the 'manage' button in the basics pref pane. It auto-senses search fields on various websites and lists them there, then you can go in and setup a trigger for it (eg, wikipedia => W, google images => gi, etc).