If you are an skilled WP developer, there is good chance that some WP framework could save you lot of time (it isn't neccesary to reinvent the wheel). Also - WP 3 have lot of features, which were earlier the reason to use frameworks (menu management, post thumbnails, custom backgrounds etc.)
However - there is a problem: most of WP frameworks is too much complex and feature-rich, there is no "middle-class" or "simple" framework. All are using hooks and lot of unusual stuff which can fetch you älot of headaches. And if you don't want to use those functions, there is no use of an framework for you...
If you are newbie, it's much smarter to start with inspecting/modifying default (or any popular http://wordpress.org/extend/themes/browse/popular/ ) WP theme to learn the basics and then to start looking for a suitable framework for you to use, if you discover you need some.
And also... Thematic is the best framework I've ever met ;)