I'm glad they've expanded on the features that are included in IE Developer Toolbar (for IE5-7/Win). However, there are a few things that I find annoying, since I have so much experience using Firebug (Firefox web developer extension).
The first thing is that in the HTML:Style view of the IE8 Dev Tools you can disable rules and/or declarations, but you can't add a new declaration. For that you have to go to the CSS view (or the HTML:Attributes view to set an inline style). I must admit, however, that adding a new style rule or changing the selector is easier in the CSS view of IE8 Dev Tools than it is in Firebug, where you have to switch to the CSS:Edit view to do so.
Another thing that is annoying is that the styles applied to the element are listed after all of the inherited styles so you have to scroll down to the bottom to see them in the HTML:Style view. I find the way Firebug lists them in reverse order so much easier to use.