Mike here. I just wanted to say that I’ve been using Twitter Bootstrap (http://twitter.github.com/bootstrap/) on a new project for a client, and I’m finding it incredibly useful. I look forward to seeing how it goes when applied to a full site, but for ease of use, and rapid prototyping, I’ve found it excellent so far.
I was very quick to jump on to the 960px grid system bandwagon that was all the rage a few years ago, and it really served its purpose at the time. Providing a solid grid-based system that had long been used in print circles, it allowed sites to be created quickly and effectively. With the explosion of mobile devices however, this rigidity was a drawback - sites were not optimised for mobile browsers and began to feel clunky compared to native apps. Now with frameworks such as Bootstrap and Foundation, sites can be designed with mobile in mind, and responsive from the beginning. Indeed, this very site was redesigned from the ground up using Skeleton (http://www.getskeleton.com/) a great, lightweight framework with responsive design in mind.