Incidentally, Safari (for mac or windows) comes with decent Firebug-like debugging functionality built-in
The Web Inspector's major shortcoming is the lack of a UI to edit the some aspects of the DOM directly (you can do it in the JS console by running DOM queries). One can't remove existing nodes or add new ones via the Web Inspector UI; nor can one add new CSS styles (although one can edit node attributes and the values of existing CSS declarations).
All of this means that even for those of us with access to Safari (i.e. not developers working on Linux) there's still a pressing need for Firebug, so if they keep breaking it with updates pretty soon everyone will be reverting to 3.0.x and the older versions which, while they had a few niggles, essentially worked.
When editing an existing rule you can add a new rule on the same line.Once you hit enter Web Inspector will put the new rule on a new line.
If there is no existing rule for an element you can cheat one in by adding 1 pixel padding via the metrics panel, which creates in inline padding style. The edit that rules or and add new rules as above.
Again, not as good as Firebug, but might be useful for some.
It looks like this is being actively worked on: https://bugs.webkit.org/show_bug.cgi?id=18224
I agree the lack of DOM editing UI is a problem.
The rest of the UI suffers from similar problems. Everything is there, it's just buried, set to a bad default, etc.
They really ought to steal from firebug more wholeheartedly.