[deanclatworthy]

It gets you going quicker than writing it yourself.

I was a heavy BS user a couple of years ago. I built my biggest project to date using BS: http://angrybirds.tumblr.com In retrospect, I wasn't really getting anything other out of it than the grid. Nowadays, I take a much more modular approach and pick an appropriate grid system, roll out my own headers as they are often heavily custom anyway, and pick some resets for the forms.