I've developed several application using bootstrap over the years and have very little bad to say about it.
I caught a question on the EmberJS dicussion digest about Semantic UI so I thought I'd give it a look. The question was specifically about not having to include the whole thing in an application, which I'm not currently worried about. I'd not use a third-party framework at all if I was optimizing for application size.
Semantic UI appears to be on par with bootstrap for intention and functionality. A grid layout, a style reset, and a set of components, ala YUI of yore. It seems to have more types of things to include in a page. Separated into categories, Semantic UI includes elements, collections (like Forms and Tables), Views (cards, feeds, stats...) and larger element sets called Modules. Lastly, it includes a set of bahaviours for form validation.
I'll look at Semantic UI a bit more along with its integration into Ember as I rough out my latest project.