I've done a few projects with Sematic UI. I think it's great for desktop based business applications. It looks slick, and has great animations. Plays nice with heaps of frameworks, I was using meteor.js
However, don't use it on mobile - it will destroy performance.
It's just a huge amount of CSS - with lots of animations and transitions. The css is somewhat optimised, it just does lots of cray shit, lots of repaints.
However, don't use it on mobile - it will destroy performance.