CSS frameworks like this can be useful and harmful at the same time, depending on what your goals are. If you need to prototype something fast they can be invaluable. But they can also become a straight jacket and require littering your layout with what are in effect, as nopassrecover mentioned, almost table cells (especially with 960.gs). All these non-semantic divs with proprietary id's break down the concept of separating content from layout. So when you want to move away from the CSS framework in the future (to say, accommodate some specialized ad unit or something) you have to go through the pain of re-writing your layouts. You have to judge how extensible the layout needs to be in the future when deciding to use these or not.