Someone please correct me if I'm wrong, but Bootstrap is also relatively unique.
We've had css frameworks before, but they were usually concerned with letting you position things along a grid structure.
The great thing about Bootstrap is that it provides you with a full complement of UI "widgets" that look reasonably good, filled in a page chocked full of examples.
We have a good designer on staff and so we were slowly starting to build something along these lines, but standardizing on Bootstrap has saved such enormous quantities of time I can't recommend it enough. I've used it on two or three projects already and it makes me happy inside.