We use semantic-ui at Highlyreco (https://www.highlyreco.com). Semantic is a CSS framework that we absolutely love. Cant recommend it enough. Semantic made it possible for 2 developers(non of us are front end focus engineers) to build out a fairly complex ui at Highlyreco. Semantic gives us the ability to iterate really fast on UI.

We probably would not exist without semantic. For earlier projects I used to use bootstrap. My opinion is that Bootstrap is good for designing landing pages and semantic is good for building user interface pages.