I've been using the nightly for a while and so far the Web Inspector has been fantastic. Good enough where I've almost completely rid myself the dependence of Firebug on FF. However the one gripe I have is that when checking the color in CSS its displayed as RGB values instead of hex. The issue is on file in their bug tracker but not sure when it'll get resolved. Not a show stopper but just annoying.
How so? the inspector in the nightlies is vastly improved over the dev preview, but if there's anything missing you should file a feature request at http://bugs.webkit.org