I have recently been investigating the use of CSS frameworks for rapid prototyping and production of webpages. In particular I have spent time implementing the 960 grid system (click here for further info) in the development of a number of websites.
On a personal level I found the framework to work very well especially when used in conjunction with a reset style sheet. This approach enabled the reduction of browser inconsistencies in things like default line heights, margins and font sizes of headings. So far I have tested with IE 6,7,8,9 Firefox, Safari and Opera. The only noticeable variation observed was the positioning of a single image on an entire test website when using IE6.
There is an on going debate surrounding the best approach to develop a webpage structure tables or div elements, of which I have practical experience of both approaches. I would say the technique used to layout a website is down to personal preference and that both approaches can work well if implemented correctly. Ultimately website ranking algorithms have a greater focus on website content as opposed to the sites script structure, so therefore if a websites structure is well defined then the majority of resources should be aimed at the quality of website content.