Bootstrap's initial intention was both not to lose time with webdesign when building back office applications and to have a consistant design in back office applications.

This is the reason why I think it's a great tool for any back office need, but not so great for front.

To me, Bootstrap feels like it's supposed to be used as-is. I love Bootstrap, it saves be a lot of time. But when I need a framework for front, I much prefer Foundation, which was meant to facilitate designing websites for Zurb's clients.