CSS frameworks are simply a solution to problems web designers experience on a daily basis, enabling them to produce clean, agile, cross-browser code. Even though these tools have their own issues to contend with, they can serve as an excellent model to produce higher quality, more portable, and reusable CSS.
From a nice article examining the strengths of CSS frameworks such as:
- 960 Grid System
- YUI Grids
- and others
Read: The Right Frame of Mind: Applying the Lessons of CSS Frameworks » SitePoint